[libcxx] Implement c++2a char8_t input/output of std::filesystem::path

This implements the std::filesystem parts of P0482 (which is already
marked as in progress), and applies the actions that are suggested
in P1423.

Differential Revision: https://reviews.llvm.org/D90222

GitOrigin-RevId: 6be11e35d5397ae1c117eb840a969585fdd7d08d
9 files changed