[clang] [Clang] Add release note for pointer overflow optimization change (PR #122462)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem approved this pull request.

The current version lgtm by the way

Maybe we should also put a blurb in the main llvm release notes and link to this one.

https://github.com/llvm/llvm-project/pull/122462