re PR libstdc++/64441 (A match_results returns an incorrect sub_match if the sub_match::matched is false)

	PR libstdc++/64441
	* include/bits/regex.h (match_results<>::size,
	match_results<>::position, match_results<>::str,
	match_results<>::operator[], match_results<>::prefix,
	match_results<>::suffix, match_results<>::end,
	match_results<>::_M_resize, match_results<>::_M_unmatched_sub,
	match_results<>::_M_prefix, match_results<>::_M_suffix): Remove
	global __unmatched_sub. Add unmatched submatch as part of
	match_results.
	* include/bits/regex.tcc (__regex_algo_impl<>, regex_replace<>,
	regex_iterator<>::operator++): Adjust to use match_results::_M_prefix.
	* testsuite/28_regex/match_results/out_of_range_submatches.cc:
	New testcases.

From-SVN: r221330