go/types: use Identical to verify type identity in the Context map

We don't have guarantees that our type hash is perfect, and in fact
fuzzing found cases where identical types hashed to different values. In
case non-identical types hash to the same value, we should ensure that
we de-duplicate using Identical.

Adjust the type map to keep a slice of distinct type identities, so that
we can guarantee that type identity is preserved by de-duplication.

To allow look-up of instances by their identity, before they are
actually instantiated, add a Context.lookup method that accepts origin
type and type arguments. Replace the multi-function typeForHash method
with an update method that requires its argument be non-nil.

Change-Id: I8fe6fb2955f508db608161b7285b02d0a2fa0e46
Reviewed-on: https://go-review.googlesource.com/c/go/+/362798


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>