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

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

> Hey @Maetveis, I've addressed the conflicts 👍

Thanks, ping @AaronBallman or @erichkeane I would appreciate a second look from someone closer to frontend.  

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