go/types, types2: fix bug in types2.MissingMethod

Because Checker.missingMethod also looks up methods matching
matching case-folded names, when Checker.missingMethod returns
an alternative method, that method does not automatically have
the wrong type. It may be a method with a different name.

Adjust types2.MissingMethod to check the alternative method
name before reporting a wrong type.

Add API test that verifies (now correct) behavior for this
case.

Ported the code also to go/types, though it was not a bug
there yet because looking up with case-folding is not yet
enabled.

Change-Id: Iaa48808535c9265a9879338ea666c6c021e93a2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/382634


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>