libstdc++: Implement constexpr std::unique_ptr for C++23 (P2273R3)

libstdc++: Implement constexpr std::unique_ptr for C++23 (P2273R3)

libstdc++-v3/ChangeLog:

	* include/bits/ptr_traits.h (__cpp_lib_constexpr_memory): Define
	conditionally.
	* include/bits/unique_ptr.h (__cpp_lib_constexpr_memory):
	Define for C++23.
	(default_delete, default_delete<T[]>, __uniq_ptr_impl)
	(unique_ptr, unique_ptr<T[], D>): Add constexpr to all member
	functions.
	* include/std/version (__cpp_lib_constexpr_memory): Define new
	value for C++23.
	* testsuite/20_util/unique_ptr/assign/constexpr.cc: New test.
	* testsuite/20_util/unique_ptr/comparison/constexpr.cc: New test.
	* testsuite/20_util/unique_ptr/cons/constexpr_c++20.cc: New test.
	* testsuite/20_util/unique_ptr/creation/constexpr.cc: New test.
	* testsuite/20_util/unique_ptr/modifiers/constexpr.cc: New test.
	* testsuite/20_util/unique_ptr/specialized_algorithms/constexpr.cc:
	New test.