[clang] Fix erroneous warning. (PR #79821)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for the fix! Can you add some more description to the patch summary as to what's erroneous (the summary ends up becoming the commit message, so a terse summary can be harmful for later code archeology).

Do we need a release note for this? Are you planning to cherry-pick this to the 18.x branch?

https://github.com/llvm/llvm-project/pull/79821