cmd/compile/internal/types2: use Identical to verify type identity in the Context map

cmd/compile/internal/types2: use Identical to verify type identity in the Context map

This is a clean port of CL 362798 from go/types to types2,
with an additional comment adjustment in types2 and go/types.

Change-Id: Ifa3d11f512f794f8ae2b6aca50b625a4a44672de
Reviewed-on: https://go-review.googlesource.com/c/go/+/364135
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>