[libc++][format] Fixes formatting code units as integers. (#73396)

This paper was voted in as a DR, so it's retroactively enabled back to
C++20; the C++ version that introduced std::format.

Implements:
- P2909R4 Fix formatting of code units as integers (Dude, where’s my
``char``?)
GitOrigin-RevId: 16b8c9608f2081c4f5a9beab6432c4aed842f2f8
12 files changed