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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

> `-emit-pch -o "%t.pch" %t/From/../From/B.h`

> `'could not find file 'D:\llvm-project\build\tools\clang\test\VFS\Output\remap-to-fake.c.tmp\From\..\From\B.h'`

Isn't that the same issue I mentioned, that it doesn't appear to work for the main PCH input file?  If you remove everything else except this file does it make progress?  If so, fixing whatever is going wrong with the main input seems promising.

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