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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

> Build machine A with a path E:/foo/header.h which wants to make a PCH from it. We want it to remap E:/foo -> Z:/fake.

What I have in mind is that you have a VFS on build machine A that maps Z:/fake (virtual path) -> E:/foo (external path), with use-external-names: false. In principle, this should give you a PCH (+ Modules) that only refers to Z:/fake, the virtual path.  Another way to think about this is except for the VFS layer, clang would not even know those external paths exist, only about Z:/fake.

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