go/types, types2: add predicate to missingMethod signature

go/types, types2: add predicate to missingMethod signature

This allows us to use missingMethod with different type comparers,
such as the global Identical predicate, or a unifier.

Preparation for the next CL.

Change-Id: I237fd9dd7feb3708847ae6d9a112bcdd0aa1ecb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/472297
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>