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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

@sheredom I think I'm missing something. What's the actual failure? Setting `-working-directory` causes `FileManager` to prepend to paths the configured directory, so the VFS lookups would then be absolute paths not relative.  I see the driver checks if the path in `-working-directory` exists, but that's using `VFS->setCurrentWorkingDirectory` which ought to work if that's the `RedirectingFileSystem`, which is what made me wonder if the issue is that the VFS isn't setup yet at that point or something.

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