re PR libstdc++/64302 (The match_results::cbegin()/cend() return incorrect results)

re PR libstdc++/64302 (The match_results::cbegin()/cend() return incorrect results)

	PR libstdc++/64302
	PR libstdc++/64303
	* include/bits/regex.h (match_results::cbegin, match_results::cend,
	regex_token_iterator::regex_token_iterator,
	regex_token_iterator::_M_normalize_result): Fix match_results cbegin
	and cend and regex_token_iterator::_M_result invariant.
	* include/bits/regex.tcc: Fix regex_token_iterator::_M_result invariant.
	* testsuite/28_regex/iterators/regex_token_iterator/64303.cc: Testcase.

From-SVN: r218810