[libc++] Implement LWG3369, tweak CTAD for std::span.

The original bug doesn't reproduce on Clang, allegedly because of
https://bugs.llvm.org/show_bug.cgi?id=44484
We already test STL's exact test case, in "span.cons/deduct.pass.cpp",
which I'm touching just for the heck of it.

Differential Revision: https://reviews.llvm.org/D111838