[clang] [clang-tools-extra] [NFC] Explicitly pass a VFS when creating DiagnosticsEngine (PR #115852)

[Boaz Brickner via cfe-commits]

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

Thanks for fixing!
It might be worth understanding whether this should have been caught by some test.
We might want to add a test or a check that would catch this.

https://github.com/llvm/llvm-project/pull/115852