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

[Neil Henning via cfe-commits]

sheredom 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.

Oh sorry I wasn't grokking what you were meaning. So just to be super duper clear - you mean that I should, when using this external names stuff, make it rewrite that path in the PCH production to respect the VFS?

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