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

[Neil Henning via cfe-commits]

sheredom wrote:

Ok already need some advice after hacking!

- `-working-directory` is only used by the **non** `-cc1` clang driver path.
- But the VFS is only initialized way deep once you are in the `-cc1` path.
- The problem is that just after `-working-directory` is set, we then use it to query the VFS to ensure that include directories are correct - which actually seems like it might be a bug too because these could also be paths that will get remapped via a vfs-overlay!

Is the correct fix to basically defer all the actions that could refer to paths until we have definitely set the VFS?

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