As mentioned here:

  https://sourceware.org/gdb/wiki/Internals%20GDB-C-Coding-Standards#Avoid_non-const_reference_parameters.2C_use_pointers_instead

we prefer to avoid non-const references.  This patch changes the
non-const references I could find in the compile/ directory, either by
making them rvalue-reference (&&) or changing them to pointers.

I'd say all the changes are pretty obvious, except the one in
compile_cplus_instance::enter_scope which might require more attention.

gdb/ChangeLog:

	* compile/compile-c.h (generate_c_for_variable_locations):
	Change reference to pointer.
	* compile/compile-c-support.c (compile_program) <compute>:
	Likewise.
	* compile/compile-c-symbols.c (generate_vla_size): Likewise.
	(generate_c_for_for_one_variable): Likewise
	(generate_c_for_variable_locations): Likewise
	* compile/compile-c-types.c (compile_c_instance::convert_type):
	Likewise
	* compile/compile-cplus-symbols.c (convert_one_symbol):
	std::move the scope passed to enter_scope.
	* compile/compile-cplus-types.c
	(compile_cplus_instance::enter_scope): Make parameter
	rvalue-reference.
	(compile_cplus_instance::new_scope): Change reference to
	pointer.
	(compile_cplus_instance::convert_type): Likewise
	(compile_cplus_convert_typedef): std::move the scope passed to
	enter_scope.
	(compile_cplus_convert_struct_or_union): Likewise.
	(compile_cplus_convert_enum): Likewise.
	(compile_cplus_convert_namespace): Likewise.
	* compile/compile-cplus.h (compile_cplus_instance)
	<enter_scope>: Make parameter rvalue-reference.
	* compile/compile-internal.h (compile_instance)
	<get_cached_type>: Likewise
	* compile/compile-loc2c.c (push): Likewise
	(pushf): Likewise
	(unary): Likewise
	(binary): Likewise
	(print_label): Likewise
	(pushf_register_address): Likewise
	(pushf_register): Likewise
	(do_compile_dwarf_expr_to_c): Likewise
	(compile_dwarf_expr_to_c): Likewise
	(compile_dwarf_bounds_to_c): Likewise
	* compile/compile.c (compile_instance::get_cached_type):
	Likewise
	* compile/compile.h (compile_dwarf_expr_to_c): Likewise.
	(compile_dwarf_bounds_to_c): Likewise
	* dwarf2loc.c (locexpr_generate_c_location): Likewise.
	(dwarf2_compile_property_to_c): Likewise
	* dwarf2loc.h (dwarf2_compile_property_to_c): Likewise
	* symtab.h (struct symbol_computed_ops) <generate_c_location>:
	Likewise