libstdc++: Add valid range assertions to std::basic_regex [PR89927]

This adds some debug assertions to basic_regex. They don't actually
diagnose the error in the PR yet, but I have another patch to make them
more effective.

Also change the __glibcxx_assert(false) consistency checks to include a
string literal that tells the user a bit more about why the process
aborted. We could consider adding a __glibcxx_bug or
__glibcxx_internal_error macro for this purpose, but ideally we'll never
hit such bugs anyway so it shouldn't be needed.

libstdc++-v3/ChangeLog:

	PR libstdc++/89927
	* include/bits/regex.h (basic_regex(const _Ch_type*, size_t)):
	Add __glibcxx_requires_string_len assertion.
	(basic_regex::assign(InputIterator, InputIterator)): Add
	__glibcxx_requires_valid_range assertion.
	* include/bits/regex_scanner.tcc (_Scanner::_M_advance())
	(_Scanner::_M_scan_normal()): Use string literal in assertions.