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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

In principle we could add this test as an xfail to make sure we have this scenario when unloading support is up to that stage. 

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