[libc++] Implement P1423R3 (char8_t backward compatibility remediation)

Reviewed By: Mordante, #libc

Spies: h-vetinari, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D129195

GitOrigin-RevId: 309aed306817e5b1a302d8f757231c734cf71370
16 files changed