[libc++][docs] Mark LWG3274 as complete

Mark LWG3274 as complete. The feature test macro `__cpp_lib_span` was added in
`6d2599e4`.

https://wg21.link/p1024 mentions marking `span:::empty()` with
`[[nodiscard]]` which is not done yet. So, do that and add tests.

Reviewed By: ldionne, Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D111516