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

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> What's the use-case for this? Isn't this equivalent to adding an `-ivfsoverlay` to the compilation command?

This is for a better swift dependency scanning. It is not the same as vfsoverlay since the file will be virtual and not existing on disk. 

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