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.