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

[Neil Henning via cfe-commits]

sheredom wrote:

So we've been digging a bit further - I think its the `ResourceDir` path that is getting messed up. Even with the VFS it is using the location of clang to get the clang headers, which don't then get remapped. Will try and make a test for it.

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