[dsymutil] Thread the VFS through dsymutil (NFC)

This patch threads the virtual file system through dsymutil.

Currently there is no good way to find out exactly what files are
necessary in order to reproduce a dsymutil link, at least not without
knowledge of how dsymutil's internals.  My motivation for this change is
to add lightweight "reproducers" that automatically gather the input
object files through the FileCollectorFileSystem. The files together
with the YAML mapping will allow us to transparently reproduce a
dsymutil link, even without having to mess with the OSO path prefix.

Differential revision: https://reviews.llvm.org/D79376