[clang] Exclude RedirectingFileSystem with null OverlayFileDir in VFSUsage (PR #128267)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

Hmm, I think this is fine, although it would be nice if there was a tighter connection between the `-ivfsoverlay` options and this. I don't think that needs to happen here though.

https://github.com/llvm/llvm-project/pull/128267