regex.tcc: Handle regex_constants::__polynomial.

regex.tcc: Handle regex_constants::__polynomial.

	* include/bits/regex.tcc: Handle regex_constants::__polynomial.
	* include/bits/regex_automaton.tcc: Throw exception when parsing
	back-reference with flag __polynomial.
	* include/bits/regex_constants.h: Add extension flag
	syntax_option_type __polynomial.
	* bits/regex_executor.tcc: Still let BFS process ECMAScript.
	Alternative operation will be fixed in the coming refactoring.
	* testsuite/28_regex/algorithms/regex_search/61424.cc: Turn
	loose match_search_debug to use DFS only.

From-SVN: r222500