libstdc++: Fix testsuite warnings about new C++23 deprecations

With the recent change to deprecate std::aligned_storage and
std::aligned_union we need to adjust some tests that now fail with
-std=c++23.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Add diagnostic pragmas around
	references to deprecated std::aligned_storage and
	std::aligned_union traits.
	* testsuite/20_util/aligned_storage/requirements/alias_decl.cc:
	Add dg-warning for et c++23.
	* testsuite/20_util/aligned_storage/requirements/explicit_instantiation.cc:
	Likewise.
	* testsuite/20_util/aligned_storage/value.cc: Likewise.
	* testsuite/20_util/aligned_union/1.cc: Likewise.
	* testsuite/20_util/aligned_union/requirements/alias_decl.cc:
	Likewise.