[clang] [clang][Sema] Always clear UndefinedButUsed (PR #73955)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

I think I understand. @AaronBallman from what concerns me that change seems fine it'd be hard to add a test for it right now. Do you have any concerns?

https://github.com/llvm/llvm-project/pull/73955