ClangImporter: permit redundancy in the Generate PCM job
`-Xcc` flags are internal to the C compiler; we have `-vfsoverlay` now
which directly passes the VFS overlay. Because the VFS overlay flags are
automatically passed along to the clang scanner, we need to pass along
`-vfsoverlay` to the Swift frontend to ensure that any VFS overlays are
honoured. Most likely this was a workaround for `-vfsoverlay` not
getting propagated originally and when the restriction was lifted the
exercised codepath was not heavily tested.
