go/types, types2: use a comparer struct to control the identical predicate

go/types, types2: use a comparer struct to control the identical predicate

This makes it easier to configure the behavior of identical: we can
simply add fields to the comparer instead of adding more parameters
to identical.

Change-Id: I9a6f5451b3ee5c37e71486060653c5a6e8f24304
Reviewed-on: https://go-review.googlesource.com/c/go/+/464937
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>