libstdc++: Define std::expected for C++23 (P0323R12)

Because this adds a new class template called std::unexpected, we have
to stop declaring the std::unexpected() function (which was deprecated
in C++11 and removed in C++17).

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in: Add new header.
	* include/Makefile.am: Likewise.
	* include/Makefile.in: Regenerate.
	* include/precompiled/stdc++.h: Add new header.
	* include/std/version (__cpp_lib_expected): Define.
	* libsupc++/exception [__cplusplus > 202002] (unexpected)
	(unexpected_handler, set_unexpected): Do not declare for C++23.
	* include/std/expected: New file.
	* testsuite/20_util/expected/assign.cc: New test.
	* testsuite/20_util/expected/cons.cc: New test.
	* testsuite/20_util/expected/illformed_neg.cc: New test.
	* testsuite/20_util/expected/observers.cc: New test.
	* testsuite/20_util/expected/requirements.cc: New test.
	* testsuite/20_util/expected/swap.cc: New test.
	* testsuite/20_util/expected/synopsis.cc: New test.
	* testsuite/20_util/expected/unexpected.cc: New test.
	* testsuite/20_util/expected/version.cc: New test.