go/types, types2: record all type instances, even duplicates

Due to instance de-duplication, we were failing to record some type
instances in types.Info.Instances. Fix this by moving the instance
recording out of the resolver.

Fixes #51494

Change-Id: Iddd8989307d95886eedb321efa4ab98cd2b3573a
Reviewed-on: https://go-review.googlesource.com/c/go/+/390041


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