[clang] [clang][deps] Use the caching VFS even in the 'preprocess' mode (PR #168970)

[Qiongsi Wu via cfe-commits]

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

This change by itself LGTM! Thanks! 

I have one question about how we setup the `DependencyScanningAction`. 

https://github.com/llvm/llvm-project/pull/168970