[libc++][format] Implement format-string.

Implements the compile-time checking of the formatting arguments.

Completes:
- P2216 std::format improvements

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D121530

GitOrigin-RevId: aed5ddf8d09762f3adb80dc842239ad67ed2e301