LWG3403 specifies that `ranges::ssize` should work on non-range objects
for which `ranges::size` is valid. This has been true in libc++ since
6f1b10d, which first implemented `ranges::ssize` for libc++13. The tests
introduced by that commit are sufficient to verify compliance since they
test calling `ranges::ssize` with various non-range arguments.
In addition to marking LWG3403 as complete, this also updates the status
page entry for P1970R2, the paper that added `ranges::ssize`. P1970R2 was
previously marked complete in libc++15 as an approximation; however, it was
actually complete in libc++13.
Resolves#104280
inquisitivecrystal
23b3f21fd0