PR libstdc++/71500 restore C++11 compatibility in <regex>

PR libstdc++/71500 restore C++11 compatibility in <regex>

	PR libstdc++/71500
	* include/bits/regex_executor.tcc
	(_Backref_matcher<BidIt, regex_traits<C>>::_M_apply): Use
	std::__equal4 instead of C++14 4-iterator overloads of std::equal.
	* include/bits/stl_algobase.h (__equal4): New functions implementing
	4-iterator overloads of std::equal for use in C++11.
	(equal(It1, It1, It2, It2), equal(It1, It1, It2, It2, BinaryPred)):
	Move function bodies to new __equal4 functions.
	* testsuite/28_regex/simple_c++11.cc: New.

From-SVN: r252981