libstdc++: Replace std::__to_address in C++20 branch in <string>

As noted by Patrick, r15-4546-g85e5b80ee2de80 should have changed the
usage of std::__to_address to std::to_address in the C++20-specific
branch that works on types satisfying std::contiguous_iterator.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (assign(Iter, Iter)): Call
	std::to_address instead of __to_address.

Reviewed-by: Patrick Palka <ppalka@redhat.com>