libstdc++: Add comparisons to std::default_sentinel_t (LWG 3719)

This library defect was recently approved for C++23.

libstdc++-v3/ChangeLog:

	* include/bits/fs_dir.h (directory_iterator): Add comparison
	with std::default_sentinel_t. Remove redundant operator!= for
	C++20.
	* (recursive_directory_iterator): Likewise.
	* include/bits/iterator_concepts.h [!__cpp_lib_concepts]
	(default_sentinel_t, default_sentinel): Define even if concepts
	are not supported.
	* include/bits/regex.h (regex_iterator): Add comparison with
	std::default_sentinel_t. Remove redundant operator!= for C++20.
	(regex_token_iterator): Likewise.
	(regex_token_iterator::_M_end_of_seq()): Add noexcept.
	* testsuite/27_io/filesystem/iterators/lwg3719.cc: New test.
	* testsuite/28_regex/iterators/regex_iterator/lwg3719.cc:
	New test.
	* testsuite/28_regex/iterators/regex_token_iterator/lwg3719.cc:
	New test.