go/types, types2: consistently use _UnsupportedFeature error code

go/types, types2: consistently use _UnsupportedFeature error code

Change-Id: Ie880871bb855e1c1f6e543508bdc7dd415451ba3
Reviewed-on: https://go-review.googlesource.com/c/go/+/429735
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>