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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Overall, I think this is a *fantastic* idea for diagnostic readability, thank you for the patch! Please be sure to add a release note so users known to expect the changes. The Windows precommit CI failure is unrelated and can be ignored.

Once you think you're ready, be sure to move the PR out of draft status so reviewers remember to give it a proper review. My initial glance didn't spot any issues though.

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