[libc++] [C++20] [P0482] Add missing tests and synopses for char8_t.

Left to finish P0482:
* <cuchar> header.
* Parts of <memory_resource> concerning char8_t. Also, tests for hash<pmr::*string>.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D99184