[libc++] Add a test for std::ssize's SFINAE

Inspired by https://reviews.llvm.org/D120684#inline-1157644 and
subsequent LWG discussion. See http://wg21.link/LWG3207 for
additional context.

Differential Revision: https://reviews.llvm.org/D121154



Co-authored-by: Louis Dionne <ldionne.2@gmail.com>