[clang] Make PCH's respect any VFS specified. (PR #106577)

[Neil Henning via cfe-commits]

sheredom wrote:

Ok by specifying `-resource-dir=Z:/resource` and adding it a VFS made that work. But we are seeing a single stray original path in the source files that we think is related to the working directory. We tried setting `-working-directory=Z:/working`, but the VFS requires that this is a real path.

Do you think we could add an overload for `setCurrentWorkingDirectory` on the actual `llvm::VirtualFileSystem` so that it will try lookup the overlay first?

https://github.com/llvm/llvm-project/pull/106577