Eenable -Winvalid-memory-order for C++ [PR99612].

Resolves:
PR middle-end/99612 - Remove "#pragma GCC system_header" from atomic file to warn on incorrect memory order

gcc/ChangeLog:

	PR middle-end/99612
	* builtins.c (get_memmodel): Move warning code to
	gimple-ssa-warn-access.cc.
	(expand_builtin_atomic_compare_exchange): Same.
	(expand_ifn_atomic_compare_exchange): Same.
	(expand_builtin_atomic_load): Same.
	(expand_builtin_atomic_store): Same.
	(expand_builtin_atomic_clear): Same.
	* doc/extend.texi (__atomic_exchange_n): Update valid memory
	models.
	* gimple-ssa-warn-access.cc (memmodel_to_uhwi): New function.
	(struct memmodel_pair): New struct.
	(memmodel_name): New function.
	(pass_waccess::maybe_warn_memmodel): New function.
	(pass_waccess::check_atomic_memmodel): New function.
	(pass_waccess::check_atomic_builtin): Handle memory model.
	* input.c (expansion_point_location_if_in_system_header): Return
	original location if expansion location is in a system header.

gcc/testsuite/ChangeLog:

	PR middle-end/99612
	* c-c++-common/pr83059.c: Adjust text of expected diagnostics.
	* gcc.dg/atomic-invalid-2.c: Same.
	* gcc.dg/atomic-invalid.c: Same.
	* c-c++-common/Winvalid-memory-model.c: New test.
	* g++.dg/warn/Winvalid-memory-model-2.C: New test.
	* g++.dg/warn/Winvalid-memory-model.C: New test.