This removes the ALL_OBJFILES_SAFE macro, replacing the uses with
ranged for loops.

gdb/ChangeLog
2019-01-09  Tom Tromey  <tom@tromey.com>

	* common/next-iterator.h (next_adapter): Add Iterator template
	parameter.
	* objfiles.h (ALL_OBJFILES_SAFE): Remove.
	(class all_objfiles_safe): New.
	* jit.c (jit_inferior_exit_hook): Use all_objfiles_safe.
	* objfiles.c (put_objfile_before): Update comment.
	(add_separate_debug_objfile): Likewise.
	(free_all_objfiles): Use all_objfiles_safe.
	(objfile_purge_solibs): Likewise.