re PR libstdc++/61424 (std::regex matches right to left, not leftmost longest)

re PR libstdc++/61424 (std::regex matches right to left, not leftmost longest)

	PR libstdc++/61424
	* include/bits/regex.tcc (__regex_algo_impl<>): Use DFS for ECMAScript,
	not just regex containing back-references.
	* include/bits/regex_compiler.tcc (_Compiler<>::_M_disjunction):
	exchange _M_next and _M_alt for alternative operator,
	making matching from left to right.
	* include/bits/regex_executor.h (_State_info<>::_M_get_sol_pos):
	Add position tracking fom DFS.
	* include/bits/regex_executor.tcc (_Executor<>::_M_main_dispatch,
	_Executor<>::_M_dfs): Likewise.
	* include/bits/regex_scanner.h: Remove unused enum entry.
	* testsuite/28_regex/algorithms/regex_search/61424.cc: New
	testcase from PR.

From-SVN: r212184