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

[Neil Henning via cfe-commits]

sheredom wrote:

So you can see [in Driver.cpp:1292](https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/Driver.cpp#L1292) that it will pass on any set `-working-directory` option to the VFS, but at this point the VFS is a `RealFileSystem`, which means it gets to [VirtualFileSystem.cpp:350](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Support/VirtualFileSystem.cpp#L350) and fails because the folder isn't on the filesystem (because it is a fake path!).

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