This change marks the overlay for a saved file as "on disk".
This will reduce the number of overlays we provide to go/packages, which
can be expensive.
Updates golang/go#31796, golang/go#32457
Change-Id: I8e69503ab80bba29caf4e42491d87e643bf17f1a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181123
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Peter Weinberger <pjw@google.com>