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

[Ben Langmuir via cfe-commits]

benlangmuir wrote:

Right, that's what I meant by "Is it trying to set it too early before the VFS is created or something?".  We could make the driver setup the VFS like the frontend would and do that before checking the -working-directory; not sure if there's a reason we don't do that already.  Or maybe we could ignore the failure to set the working directory in the driver and check it in the frontend? That might cause some poor diagnostics if the driver is accessing relative paths and the working directory wasn't set successfully

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