This is more preparation bits for multi-target support.

In a multi-target scenario, we need to address the case of different
processes/threads running on different targets that happen to have the
same PID/PTID.  E.g., we can have both process 123 in target 1, and
process 123 in target 2, while they're in reality different processes
running on different machines.  Or maybe we've loaded multiple
instances of the same core file.  Etc.

To address this, in my WIP multi-target branch, threads and processes
are uniquely identified by the (process_stratum target_ops *, ptid_t)
and (process_stratum target_ops *, pid) tuples respectively.  I.e.,
each process_stratum instance has its own thread/process number space.

As you can imagine, that requires passing around target_ops * pointers
in a number of functions where we're currently passing only a ptid_t
or an int.  E.g., when we look up a thread_info object by ptid_t in
find_thread_ptid, the ptid_t alone isn't sufficient.

In many cases though, we already have the thread_info or inferior
pointer handy, but we "lose" it somewhere along the call stack, only
to look it up again by ptid_t/pid.  Since thread_info or inferior
objects know their parent target, if we pass around thread_info or
inferior pointers when possible, we avoid having to add extra
target_ops parameters to many functions, and also, we eliminate a
number of by ptid_t/int lookups.

So that's what this patch does.  In a bit more detail:

- Changes a number of functions and methods to take a thread_info or
  inferior pointer instead of a ptid_t or int parameter.

- Changes a number of structure fields from ptid_t/int to inferior or
  thread_info pointers.

- Uses the inferior_thread() function whenever possible instead of
  inferior_ptid.

- Uses thread_info pointers directly when possible instead of the
  is_running/is_stopped etc. routines that require a lookup.

- A number of functions are eliminated along the way, such as:

  int valid_gdb_inferior_id (int num);
  int pid_to_gdb_inferior_id (int pid);
  int gdb_inferior_id_to_pid (int num);
  int in_inferior_list (int pid);

- A few structures and places hold a thread_info pointer across
  inferior execution, so now they take a strong reference to the
  (refcounted) thread_info object to avoid the thread_info pointer
  getting stale.  This is done in enable_thread_stack_temporaries and
  in the infcall.c code.

- Related, there's a spot in infcall.c where using a RAII object to
  handle the refcount would be handy, so a gdb::ref_ptr specialization
  for thread_info is added (thread_info_ref, in gdbthread.h), along
  with a gdb_ref_ptr policy that works for all refcounted_object types
  (in common/refcounted-object.h).

gdb/ChangeLog:
2018-06-21  Pedro Alves  <palves@redhat.com>

	* ada-lang.h (ada_get_task_number): Take a thread_info pointer
	instead of a ptid_t.  All callers adjusted.
	* ada-tasks.c (ada_get_task_number): Likewise.  All callers
	adjusted.
	(print_ada_task_info, display_current_task_id, task_command_1):
	Adjust.
	* breakpoint.c (watchpoint_in_thread_scope): Adjust to use
	inferior_thread.
	(breakpoint_kind): Adjust.
	(remove_breakpoints_pid): Rename to ...
	(remove_breakpoints_inf): ... this.  Adjust to take an inferior
	pointer.  All callers adjusted.
	(bpstat_clear_actions): Use inferior_thread.
	(get_bpstat_thread): New.
	(bpstat_do_actions): Use it.
	(bpstat_check_breakpoint_conditions, bpstat_stop_status): Adjust
	to take a thread_info pointer.  All callers adjusted.
	(set_longjmp_breakpoint_for_call_dummy, set_momentary_breakpoint)
	(breakpoint_re_set_thread): Use inferior_thread.
	* breakpoint.h (struct inferior): Forward declare.
	(bpstat_stop_status): Update.
	(remove_breakpoints_pid): Delete.
	(remove_breakpoints_inf): New.
	* bsd-uthread.c (bsd_uthread_target::wait)
	(bsd_uthread_target::update_thread_list): Use find_thread_ptid.
	* btrace.c (btrace_add_pc, btrace_enable, btrace_fetch)
	(maint_btrace_packet_history_cmd)
	(maint_btrace_clear_packet_history_cmd): Adjust.
	(maint_btrace_clear_cmd, maint_info_btrace_cmd): Adjust to use
	inferior_thread.
	* cli/cli-interp.c: Include "inferior.h".
	* common/refcounted-object.h (struct
	refcounted_object_ref_policy): New.
	* compile/compile-object-load.c: Include gdbthread.h.
	(store_regs): Use inferior_thread.
	* corelow.c (core_target::close): Use current_inferior.
	(core_target_open): Adjust to use first_thread_of_inferior and use
	the current inferior.
	* ctf.c (ctf_target::close): Adjust to use current_inferior.
	* dummy-frame.c (dummy_frame_id) <ptid>: Delete, replaced by ...
	<thread>: ... this new field.  All references adjusted.
	(dummy_frame_pop, dummy_frame_discard, register_dummy_frame_dtor):
	Take a thread_info pointer instead of a ptid_t.
	* dummy-frame.h (dummy_frame_push, dummy_frame_pop)
	(dummy_frame_discard, register_dummy_frame_dtor): Take a
	thread_info pointer instead of a ptid_t.
	* elfread.c: Include "inferior.h".
	(elf_gnu_ifunc_resolver_stop, elf_gnu_ifunc_resolver_return_stop):
	Use inferior_thread.
	* eval.c (evaluate_subexp): Likewise.
	* frame.c (frame_pop, has_stack_frames, find_frame_sal): Use
	inferior_thread.
	* gdb_proc_service.h (struct thread_info): Forward declare.
	(struct ps_prochandle) <ptid>: Delete, replaced by ...
	<thread>: ... this new field.  All references adjusted.
	* gdbarch.h, gdbarch.c: Regenerate.
	* gdbarch.sh (get_syscall_number): Replace 'ptid' parameter with a
	'thread' parameter.  All implementations and callers adjusted.
	* gdbthread.h (thread_info) <set_running>: New method.
	(delete_thread, delete_thread_silent): Take a thread_info pointer
	instead of a ptid.
	(global_thread_id_to_ptid, ptid_to_global_thread_id): Delete.
	(first_thread_of_process): Delete, replaced by ...
	(first_thread_of_inferior): ... this new function.  All callers
	adjusted.
	(any_live_thread_of_process): Delete, replaced by ...
	(any_live_thread_of_inferior): ... this new function.  All callers
	adjusted.
	(switch_to_thread, switch_to_no_thread): Declare.
	(is_executing): Delete.
	(enable_thread_stack_temporaries): Update comment.
	<enable_thread_stack_temporaries>: Take a thread_info pointer
	instead of a ptid_t.  Incref the thread.
	<~enable_thread_stack_temporaries>: Decref the thread.
	<m_ptid>: Delete
	<m_thr>: New.
	(thread_stack_temporaries_enabled_p, push_thread_stack_temporary)
	(get_last_thread_stack_temporary)
	(value_in_thread_stack_temporaries, can_access_registers_thread):
	Take a thread_info pointer instead of a ptid_t.  All callers
	adjusted.
	* infcall.c (get_call_return_value): Use inferior_thread.
	(run_inferior_call): Work with thread pointers instead of ptid_t.
	(call_function_by_hand_dummy): Work with thread pointers instead
	of ptid_t.  Use thread_info_ref.
	* infcmd.c (proceed_thread_callback): Access thread's state
	directly.
	(ensure_valid_thread, ensure_not_running): Use inferior_thread,
	access thread's state directly.
	(continue_command): Use inferior_thread.
	(info_program_command): Use find_thread_ptid and access thread
	state directly.
	(proceed_after_attach_callback): Use thread state directly.
	(notice_new_inferior): Take a thread_info pointer instead of a
	ptid_t.  All callers adjusted.
	(exit_inferior): Take an inferior pointer instead of a pid.  All
	callers adjusted.
	(exit_inferior_silent): New.
	(detach_inferior): Delete.
	(valid_gdb_inferior_id, pid_to_gdb_inferior_id)
	(gdb_inferior_id_to_pid, in_inferior_list): Delete.
	(detach_inferior_command, kill_inferior_command): Use
	find_inferior_id instead of valid_gdb_inferior_id and
	gdb_inferior_id_to_pid.
	(inferior_command): Use inferior and thread pointers.
	* inferior.h (struct thread_info): Forward declare.
	(notice_new_inferior): Take a thread_info pointer instead of a
	ptid_t.  All callers adjusted.
	(detach_inferior): Delete declaration.
	(exit_inferior, exit_inferior_silent): Take an inferior pointer
	instead of a pid.  All callers adjusted.
	(gdb_inferior_id_to_pid, pid_to_gdb_inferior_id, in_inferior_list)
	(valid_gdb_inferior_id): Delete.
	* infrun.c (follow_fork_inferior, proceed_after_vfork_done)
	(handle_vfork_child_exec_or_exit, follow_exec): Adjust.
	(struct displaced_step_inferior_state) <pid>: Delete, replaced by
	...
	<inf>: ... this new field.
	<step_ptid>: Delete, replaced by ...
	<step_thread>: ... this new field.
	(get_displaced_stepping_state): Take an inferior pointer instead
	of a pid.  All callers adjusted.
	(displaced_step_in_progress_any_inferior): Adjust.
	(displaced_step_in_progress_thread): Take a thread pointer instead
	of a ptid_t.  All callers adjusted.
	(displaced_step_in_progress, add_displaced_stepping_state): Take
	an inferior pointer instead of a pid.  All callers adjusted.
	(get_displaced_step_closure_by_addr): Adjust.
	(remove_displaced_stepping_state): Take an inferior pointer
	instead of a pid.  All callers adjusted.
	(displaced_step_prepare_throw, displaced_step_prepare)
	(displaced_step_fixup): Take a thread pointer instead of a ptid_t.
	All callers adjusted.
	(start_step_over): Adjust.
	(infrun_thread_ptid_changed): Remove bit updating ptids in the
	displaced step queue.
	(do_target_resume): Adjust.
	(fetch_inferior_event): Use inferior_thread.
	(context_switch, get_inferior_stop_soon): Take an
	execution_control_state pointer instead of a ptid_t.  All callers
	adjusted.
	(switch_to_thread_cleanup): Delete.
	(stop_all_threads): Use scoped_restore_current_thread.
	* inline-frame.c: Include "gdbthread.h".
	(inline_state) <inline_state>: Take a thread pointer instead of a
	ptid_t.  All callers adjusted.
	<ptid>: Delete, replaced by ...
	<thread>: ... this new field.
	(find_inline_frame_state): Take a thread pointer instead of a
	ptid_t.  All callers adjusted.
	(skip_inline_frames, step_into_inline_frame)
	(inline_skipped_frames, inline_skipped_symbol): Take a thread
	pointer instead of a ptid_t.  All callers adjusted.
	* inline-frame.h (skip_inline_frames, step_into_inline_frame)
	(inline_skipped_frames, inline_skipped_symbol): Likewise.
	* linux-fork.c (delete_checkpoint_command): Adjust to use thread
	pointers directly.
	* linux-nat.c (get_detach_signal): Likewise.
	* linux-thread-db.c (thread_from_lwp): New 'stopped' parameter.
	(thread_db_notice_clone): Adjust.
	(thread_db_find_new_threads_silently)
	(thread_db_find_new_threads_2, thread_db_find_new_threads_1): Take
	a thread pointer instead of a ptid_t.  All callers adjusted.
	* mi/mi-cmd-var.c: Include "inferior.h".
	(mi_cmd_var_update_iter): Update to use thread pointers.
	* mi/mi-interp.c (mi_new_thread): Update to use the thread's
	inferior directly.
	(mi_output_running_pid, mi_inferior_count): Delete, bits factored
	out to ...
	(mi_output_running): ... this new function.
	(mi_on_resume_1): Adjust to use it.
	(mi_user_selected_context_changed): Adjust to use inferior_thread.
	* mi/mi-main.c (proceed_thread): Adjust to use thread pointers
	directly.
	(interrupt_thread_callback): : Adjust to use thread and inferior
	pointers.
	* proc-service.c: Include "gdbthread.h".
	(ps_pglobal_lookup): Adjust to use the thread's inferior directly.
	* progspace-and-thread.c: Include "inferior.h".
	* progspace.c: Include "inferior.h".
	* python/py-exitedevent.c (create_exited_event_object): Adjust to
	hold a reference to an inferior_object.
	* python/py-finishbreakpoint.c (bpfinishpy_init): Adjust to use
	inferior_thread.
	* python/py-inferior.c (struct inferior_object): Give the type a
	tag name instead of a typedef.
	(python_on_normal_stop): No need to check if the current thread is
	listed.
	(inferior_to_inferior_object): Change return type to
	inferior_object.  All callers adjusted.
	(find_thread_object): Delete, bits factored out to ...
	(thread_to_thread_object): ... this new function.
	* python/py-infthread.c (create_thread_object): Use
	inferior_to_inferior_object.
	(thpy_is_stopped): Use thread pointer directly.
	(gdbpy_selected_thread): Use inferior_thread.
	* python/py-record-btrace.c (btpy_list_object) <ptid>: Delete
	field, replaced with ...
	<thread>: ... this new field.  All users adjusted.
	(btpy_insn_or_gap_new): Drop const.
	(btpy_list_new): Take a thread pointer instead of a ptid_t.  All
	callers adjusted.
	* python/py-record.c: Include "gdbthread.h".
	(recpy_insn_new, recpy_func_new): Take a thread pointer instead of
	a ptid_t.  All callers adjusted.
	(gdbpy_current_recording): Use inferior_thread.
	* python/py-record.h (recpy_record_object) <ptid>: Delete
	field, replaced with ...
	<thread>: ... this new field.  All users adjusted.
	(recpy_element_object) <ptid>: Delete
	field, replaced with ...
	<thread>: ... this new field.  All users adjusted.
	(recpy_insn_new, recpy_func_new): Take a thread pointer instead of
	a ptid_t.  All callers adjusted.
	* python/py-threadevent.c: Include "gdbthread.h".
	(get_event_thread): Use thread_to_thread_object.
	* python/python-internal.h (struct inferior_object): Forward
	declare.
	(find_thread_object, find_inferior_object): Delete declarations.
	(thread_to_thread_object, inferior_to_inferior_object): New
	declarations.
	* record-btrace.c: Include "inferior.h".
	(require_btrace_thread): Use inferior_thread.
	(record_btrace_frame_sniffer)
	(record_btrace_tailcall_frame_sniffer): Use inferior_thread.
	(get_thread_current_frame): Use scoped_restore_current_thread and
	switch_to_thread.
	(get_thread_current_frame): Use thread pointer directly.
	(record_btrace_replay_at_breakpoint): Use thread's inferior
	pointer directly.
	* record-full.c: Include "inferior.h".
	* regcache.c: Include "gdbthread.h".
	(get_thread_arch_regcache): Use the inferior's address space
	directly.
	(get_thread_regcache, registers_changed_thread): New.
	* regcache.h (get_thread_regcache(thread_info *thread)): New
	overload.
	(registers_changed_thread): New.
	(remote_target) <remote_detach_1>: Swap order of parameters.
	(remote_add_thread): <remote_add_thread>: Return the new thread.
	(get_remote_thread_info(ptid_t)): New overload.
	(remote_target::remote_notice_new_inferior): Use thread pointers
	directly.
	(remote_target::process_initial_stop_replies): Use
	thread_info::set_running.
	(remote_target::remote_detach_1, remote_target::detach)
	(extended_remote_target::detach): Adjust.
	* stack.c (frame_show_address): Use inferior_thread.
	* target-debug.h (target_debug_print_thread_info_pp): New.
	* target-delegates.c: Regenerate.
	* target.c (default_thread_address_space): Delete.
	(memory_xfer_partial_1): Use current_inferior.
	(target_detach): Use current_inferior.
	(target_thread_address_space): Delete.
	(generic_mourn_inferior): Use current_inferior.
	* target.h (struct target_ops) <thread_address_space>: Delete.
	(target_thread_address_space): Delete.
	* thread.c (init_thread_list): Use ALL_THREADS_SAFE.  Use thread
	pointers directly.
	(delete_thread_1, delete_thread, delete_thread_silent): Take a
	thread pointer instead of a ptid_t.  Adjust all callers.
	(ptid_to_global_thread_id, global_thread_id_to_ptid): Delete.
	(first_thread_of_process): Delete, replaced by ...
	(first_thread_of_inferior): ... this new function.  All callers
	adjusted.
	(any_thread_of_process): Rename to ...
	(any_thread_of_inferior): ... this, and take an inferior pointer.
	(any_live_thread_of_process): Rename to ...
	(any_live_thread_of_inferior): ... this, and take an inferior
	pointer.
	(thread_stack_temporaries_enabled_p, push_thread_stack_temporary)
	(value_in_thread_stack_temporaries)
	(get_last_thread_stack_temporary): Take a thread pointer instead
	of a ptid_t.  Adjust all callers.
	(thread_info::set_running): New.
	(validate_registers_access): Use inferior_thread.
	(can_access_registers_ptid): Rename to ...
	(can_access_registers_thread): ... this, and take a thread
	pointer.
	(print_thread_info_1): Adjust to compare thread pointers instead
	of ptids.
	(switch_to_no_thread, switch_to_thread): Make extern.
	(scoped_restore_current_thread::~scoped_restore_current_thread):
	Use m_thread pointer directly.
	(scoped_restore_current_thread::scoped_restore_current_thread):
	Use inferior_thread.
	(thread_command): Use thread pointer directly.
	(thread_num_make_value_helper): Use inferior_thread.
	* top.c (execute_command): Use inferior_thread.
	* tui/tui-interp.c: Include "inferior.h".
	* varobj.c (varobj_create): Use inferior_thread.
	(value_of_root_1): Use find_thread_global_id instead of
	global_thread_id_to_ptid.