libstdc++: Implement LWG 3403 for std::ranges::ssize

libstdc++: Implement LWG 3403 for std::ranges::ssize

I already changed the constraints for ranges::ssize to use ranges::size,
this implements the rest of LWG 3403, so that the returned type is the
signed type corresponding to the result of ranges::size.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (_SSize): Return the result of
	ranges::size converted to the wider of make-signed-like-t<S> and
	ptrdiff_t, rather than the ranges different type.
	* testsuite/std/ranges/access/ssize.cc: Adjust expected result
	for an iota_view that uses an integer class type for its
	difference_type.