go/types, types2: fix implements and identical predicates

- Use the correct predicate in Checker.implements: for interfaces
  we cannot use the API Comparable because it always returns true
  for all non-type parameter interface types: Comparable simply
  answers if == and != is permitted, and it's always been permitted
  for interfaces. Instead we must use Interface.IsComparable which
  looks at the type set of an interface.

- When comparing interfaces for identity, we must also consider the
  whether the type sets have the comparable bit set.

With this change, `any` doesn't implement `comparable` anymore. This
only matters for generic functions and types, and the API functions.
It does mean that for now (until we allow type-constrained interfaces
for general non-constraint use, at some point in the future) a type
parameter that needs to be comparable cannot be instantiated with an
interface anymore.

For #50646.

Change-Id: I7e7f711bdcf94461f330c90509211ec0c2cf3633
Reviewed-on: https://go-review.googlesource.com/c/go/+/381254


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