[clang] [llvm] [clang] Use separator for large numeric values in overflow diagnostic (PR #80939)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

You should also add a release note to `clang/docs/ReleaseNotes.rst` to tell users about the improved diagnostic behavior.

Aside from some small nits, this looks really close to ready!

https://github.com/llvm/llvm-project/pull/80939