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

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

Hey, @whiteio I'm sorry the review is taking long, Aaron was off for the holidays. In the meantime it looks like some conflicts came up, are you comfortable resolving those or do you need help?

I'll ping Aaron or find some other front-end reviewer once the conflicts are resolved.

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