re PR libstdc++/64584 (basic_regex::assign breaks *this if it throws regex_error)

re PR libstdc++/64584 (basic_regex::assign breaks *this if it throws regex_error)

	PR libstdc++/64584
	PR libstdc++/64585
	* include/bits/regex.h (basic_regex<>::basic_regex,
	basic_regex<>::assign, basic_regex<>::imbue,
	basic_regex<>::swap, basic_regex<>::mark_count): Drop NFA after
	imbuing basic_regex; Make assign() transactional against exception.
	* include/bits/regex_compiler.h (__compile_nfa<>): Add back
	__compile_nfa SFINAE.
	* include/std/regex: Adjust include order to avoid __compile_nfa
	forward declaration.
	* testsuite/28_regex/basic_regex/assign/char/string.cc: New testcase.
	* testsuite/28_regex/basic_regex/imbue/string.cc: New testcase.

From-SVN: r219865