PR libstdc++/95915

PR libstdc++/95915

	PR libstdc++/95915
	* include/std/type_traits (is_literal_type, is_literal_type_v):
	Deprecate in C++17.
	* include/std/variant (_Uninitialized):
	Adjust the condition and the comment.
	* testsuite/20_util/is_literal_type/deprecated-1z.cc: New.
	* testsuite/20_util/is_literal_type/requirements/explicit_instantiation.cc:
	Adjust.
	* testsuite/20_util/is_literal_type/requirements/typedefs.cc: Likewise.
	* testsuite/20_util/is_literal_type/value.cc: Likewise.
	* testsuite/20_util/optional/constexpr/nullopt.cc:
	Use __is_literal_type directly.
	* testsuite/20_util/optional/nullopt.cc: Likewise.
	* testsuite/20_util/variable_templates_for_traits.cc: Adjust.
	* testsuite/20_util/variant/95915.cc: New.
	* testsuite/20_util/variant/compile.cc: Add new test.
	* testsuite/experimental/optional/constexpr/nullopt.cc:
	Use __is_literal_type directly.
	* testsuite/experimental/optional/nullopt.cc: Likewise.
	* testsuite/experimental/type_traits/value.cc: Adjust.
	* testsuite/util/testsuite_common_types.h:
	Use __is_literal_type directly.