go/types, types2: combine missingMethodCause with missingMethod

go/types, types2: combine missingMethodCause with missingMethod

For now this is simply a mechanical combination without any
relevant logic changes. This will make it easier to review
subsequent changes.

Change-Id: Ic2e2ae32b031d2dda0f69a08a5cc2349748bd25d
Reviewed-on: https://go-review.googlesource.com/c/go/+/472675
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>