[PATCH] Fix PR15558

[Rafael Espíndola]

>> Can't you avoid putting these variables in UnusedFileScopedDecls in the first place instead of filtering them out at the end of the TU?
>
> I wanted to keep the scope of this change relatively small, since the machinery that feeds into these diagnostics is pretty complicated and I don't have a good handle on all of it. I'm happy to broaden the scope if you think that's the right way to go.

We can do it in two patches, but if it is desirable to avoid both
warnings I think we should try as not adding the decls to
UnusedFileScopedDecls will be more efficient.

In any case, the current patch LGTM.

Cheers,
Rafael