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

[Neil Henning via cfe-commits]

sheredom wrote:

> Instead of using VFS overlays to make the AST file relocatable, have you considered making use of `adjustFilenameForRelocatableAST()` (i.e. storing relative paths to the AST file) and then setting the CWD accordingly when loading?

The problem we've got is that while we've only got a single path being relocated in the example, in practice we're actually going to remap multiple of these. We'll at least be relocating:

- The location of the checked out unreal engine folder.
- The location of the Visual Studio.
- The location of the Windows SDK.
- Maybe some proprietary platform toolchain stuff too.

And to make this work we'd have to provide different path relocations for all of them.

I'm not sure relocatable PCH would work for that case?

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