libstdc++: Reduce template instantiations in <regex>

libstdc++: Reduce template instantiations in <regex>

This moves the last two template parameters of __regex_algo_impl to be
runtime function parameters instead, so that we don't need four
different instantiations for the possible ways to call it. Most of the
function (and what it instantiates) is the same in all cases, so making
them compile-time choices doesn't really have much benefit.

Use  'if constexpr' for conditions that check template parameters, so
that when we do depend on a compile-time condition we only instantiate
what we need to.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (__regex_algo_impl): Change __policy and
	__match_mode template parameters to be function parameters.
	(regex_match, regex_search): Pass policy and match mode as
	function arguments.
	* include/bits/regex.tcc (__regex_algo_impl): Change template
	parameters to function parameters.
	* include/bits/regex_compiler.h (_RegexTranslatorBase): Use
	'if constexpr' for conditions using template parameters.
	(_RegexTranslator): Likewise.
	* include/bits/regex_executor.tcc (_Executor::_M_handle_accept):
	Likewise.
	* testsuite/util/testsuite_regex.h (regex_match_debug)
	(regex_search_debug): Move template arguments to function
	arguments.