PR libstdc++/64239
	* include/bits/regex.h (match_results<>::match_results,
	match_results<>::operator=, match_results<>::position,
	match_results<>::swap): Remove match_results::_M_in_iterator.
	Fix ctor/assign/swap.
	* include/bits/regex.tcc: (__regex_algo_impl<>,
	regex_iterator<>::operator++): Set match_results::_M_begin as
	"start position".
	* testsuite/28_regex/iterators/regex_iterator/char/
	string_position_01.cc: Test cases.

From-SVN: r218710