[llvm] Add `vfs::FileSystem` to `PassBuilder` (#160188)
Some LLVM passes need access to the filesystem to read configuration
files and similar. In some places, this is achieved by grabbing the VFS
from `PGOOptions`, but some passes don't have access to these and resort
to just calling `vfs::getRealFileSystem()`. This PR allows setting the
VFS directly on `PassBuilder` that's able to pass it down to all passes
that need it.
