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

[Neil Henning via cfe-commits]

sheredom wrote:

> > We tried setting -working-directory=Z:/working, but the VFS requires that this is a real path.
> 
> What error are you seeing if it's not? Is it trying to set it too early before the VFS is created or something?

The VFS doesn't actually override the set current working directory, so it goes down to the base file system class and that will check that the path exists on the real filesystem (which it doesn't, cause it is fake!).

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