go/types, types2: be sure to type-check wrong methods in missingMethod

go/types, types2: be sure to type-check wrong methods in missingMethod

In the case of a wrong method, we were not ensuring that it was
type-checked before passing it to funcString.

Formatting the missing method error message requires a fully set-up
signature.

Fixes #59848

Change-Id: I1467e036afbbbdd00899bfd627a945500dc709c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/494615
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>