re PR libstdc++/71500 (regex::icase only works on first character in a range)

	PR libstdc++/71500
	* include/bits/regex.h (basic_regex::basic_regex): Use ECMAScript
	when the syntax is not specified.
	* include/bits/regex_compiler.h (_RegexTranslator,
	_RegexTranslatorBase): Partially support icase in ranges.
	* include/bits/regex_compiler.tcc (_BracketMatcher::_M_apply):
	Refactor _M_apply to make the control flow easier to follow, and
	call _M_translator._M_match_range as added previously.
	* testsuite/28_regex/traits/char/icase.cc: Add new tests.
	* testsuite/28_regex/traits/char/user_defined.cc: Add new tests.

From-SVN: r243093