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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

I think it's something much earlier in clang is not using the VFS at all when looking up the main input path, so then it only knows the external path for that case.  My guess is that if that were fixed then the ASTWriter/Reader would do the right thing here.



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