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.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@218710 138bc75d-0d04-0410-961f-82ee72b054a4