re PR libstdc++/63920 (Any regular expression should not match an empty sequence if match_not_null is specified)

	PR libstdc++/63920
	* include/bits/regex_executor.h: Make _M_begin non const.
	* include/bits/regex_executor.tcc (_Executor<>::_M_search): Increase
	_M_begin in search algorithm, so that _M_begin is treated as
	"current start position" for each search iteration.
	* testsuite/28_regex/algorithms/regex_search/ecma/flags.cc: New
	testcase.

From-SVN: r218037