go/types, types2: change missingMethod to match MissingMethod signature

go/types, types2: change missingMethod to match MissingMethod signature

This simplifies the use of missingMethod and also opens the door to
further missingMethod-internal simplifications.

Change-Id: I74d9694b6fca67c4103aea04d08916a69ad0e3c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/472495
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>