[libc++][format] Exposes basic-format-string

This paper was accepted during the last plenary and is intended to be
backported to LLVM 15. When backporting the release notes in the branch
should be updated too.

Note the feature-test macro isn't updated since this will change; three
papers have updated the same macro in the same plenary.

Implements:
- P2508R1 Exposing std::basic-format-string

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D130643

(cherry picked from commit f712775d)