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

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

I tried to craft a test here, but declaration unloading in `clang-repl` is not powerful enough (yet) to show a (user-visible) consequence of the problem.

On a high level, the problem should be triggered by:
```
clang-repl> template <typename T> struct A { void f() { } };
clang-repl> A<int>().f();
clang-repl> %undo
clang-repl> A<int>().f();
```
In principle, the `%undo` should remove the implicit template instantiation of `f` which is then subsequently re-instantiated. This is currently not implemented in `clang-repl` (we just re-use the first instantiation, which is wrong in case there are more lines in between that could influence the instantiation). With debug statements, I can verify that `f` is in `UndefinedButUsed`, but `getUndefinedButUsed` filters it out.

The slightly more complex
```
clang-repl> template <typename T> struct A { void f() { } };
clang-repl> A<int>().f();
clang-repl> %undo
clang-repl> %undo
clang-repl> template <typename T> struct A { void f() { } };
clang-repl> A<int>().f();
```
ie unloading the entire class doesn't work either for the same reason, we never actually treat the instantiated member function. (Subsequently this leads to the entire `clang-repl` crashing after printing `definition with same mangled name '_ZN1AIiE1fEv' as another definition`...)

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