go/types, types2: instantiate generic alias types

go/types, types2: instantiate generic alias types

For #46477.

Change-Id: Ifa47d3ff87f67c60fa25654e54194ca8b31ea5a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/567617
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>