go/types, types2: better error when method is missing due to ambiguity

go/types, types2: better error when method is missing due to ambiguity

If a type doesn't implement an interface due to an ambiguous method,
say so in the error message instead of just reporting a missing method.

Fixes #57352.

Change-Id: I5608f893c485de578b7f86362ca6f66033653695
Reviewed-on: https://go-review.googlesource.com/c/go/+/473658
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>