[clang] [Clang] Demote mixed enumeration arithmetic error to a warning (PR #131811)

[Aaron Ballman via cfe-commits]

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

LGTM, but be sure to add a release note on the backport so users know about the change in behavior.

https://github.com/llvm/llvm-project/pull/131811