Implement LWG 2744 and LWG 2754.

	Implement LWG 2744 and LWG 2754.
	* include/std/any (any(ValueType&&)): Constrain with __is_in_place_type.
	(any(in_place_type_t<_ValueType>, _Args&&...)): Use _Decay.
	(any(in_place_type_t<_ValueType>, initializer_list<_Up>, _Args&&...)):
	Likewise.
	(emplace(_Args&&...)): Likewise.
	(emplace(initializer_list<_Up>, _Args&&...)): Likewise.
	* include/std/utility: (__is_in_place_type_impl): New.
	(__is_in_place_type): Likewise.
	* testsuite/20_util/any/assign/emplace.cc: Add tests for decaying
	emplace.
	* testsuite/20_util/any/cons/in_place.cc: Add tests for decaying
	in_place constructor.
	* testsuite/20_util/any/misc/any_cast_neg.cc: Adjust.
	* testsuite/20_util/any/requirements.cc: Add a test for
	in_place-constructing a non-default-constructible type.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@239482 138bc75d-0d04-0410-961f-82ee72b054a4