[-Wunsafe-buffer-usage][NFC] Fix Fixables filtering

We have WIP Fixables for local variables and this central part of the machinery
was dropping Fixables attached to local variables instead of keeping those and
dropping everything else.
We are in the process of rewriting our patches for emitting fixits after we
discovered a conceptual problem in our design.
That is why there's currently no tests that would've detected the issue but
that will change very shortly.