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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

I will try, but observing the consequences of this depends on unloading: Basically it happens if a declaration in `UndefinedButUsed` thas was previously defined is unloaded, which makes it undefined. For now, it's possible that for the upstream cases it's only an optimization because the data structure doesn't grow as much.

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