[clang] support relative roots to vfs overlays

This diff adds support for relative roots to VFS overlays. The directory root
will be made absolute from the current working directory and will be used to
determine the path style to use. This supports the use of VFS overlays with
remote build systems that might use a different working directory for each
compilation.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D116174