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

[Neil Henning via cfe-commits]

sheredom wrote:

> > If the goal is to put virtual paths in the PCH so that you can map them somewhere else in the consuming compiler's VFS, does the`RedirectingFileSystem` setting `'use-external-names': false` do what you need? The idea behind that setting is that we would use the virtual paths everywhere in the compiler instead of translating them to the external/on-disk path. I don't have a lot of experience with enabling that in modules/pch so maybe there's something that it doesn't handle.
> 
> I'll have a look at that and get back to you!

Ok we tested this and it doesn't work as expected - the generated PCH still has the original paths baked into it.

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