libstdc++: Simplify std::basic_regex::assign

We know that if __is_contiguous_iterator is true then we have a pointer
or a __normal_iterator that wraps a pointer, so we don't need to use
std::__to_address.

libstdc++-v3/ChangeLog:

	* include/bits/regex.h (basic_regex::assign(Iter, Iter)): Avoid
	std::__to_address by using poitner directly or using base()
	member of __normal_iterator.