libstdc++: Implement constexpr std::bitset for C++23 (P2417R2)

libstdc++: Implement constexpr std::bitset for C++23 (P2417R2)

Also add _GLIBCXX_HOSTED checks to simplify making <bitset>
freestanding in the near future.

libstdc++-v3/ChangeLog:

	* include/std/bitset (bitset): Add constexpr for C++23. Guard
	members using std::string with _GLIBCXX_HOSTED.
	* include/std/version (__cpp_lib_constexpr_bitset): Define.
	* testsuite/20_util/bitset/access/constexpr.cc: New test.
	* testsuite/20_util/bitset/cons/constexpr_c++23.cc: New test.
	* testsuite/20_util/bitset/count/constexpr.cc: New test.
	* testsuite/20_util/bitset/ext/constexpr.cc: New test.
	* testsuite/20_util/bitset/operations/constexpr_c++23.cc: New test.
	* testsuite/20_util/bitset/version.cc: New test.