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

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 commented:

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?

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