[clang] [Clang] Fix -Wunused-private-field false negative with defaulted comparison operators (PR #116871)

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

Thanks! I added @AaronBallman, as a reviewer just to make sure I didn't miss anything, as I don't consider myself an expert when it comes to frontend. @AaronBallman I think you can merge if everything looks okay, I'm pretty sure @whiteio does not have commit rights.

https://github.com/llvm/llvm-project/pull/116871