[Bug libstdc++/64239] regex_iterator::operator= should copy match_results::position

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64239

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
match_results::swap() fails to handle _M_in_iterator as well.