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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM, but please give others the time to chime in and approve.

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