[clang] [ClangScanDeps] Improve getTranslationUnitDependencies (PR #159472)

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> So Swift will provide an in-memory VFS and a command-line whose input points into the VFS?

Yes. That is the plan. 

https://github.com/llvm/llvm-project/pull/159472