go/types, types2: Named.cleanup must also handle *Alias types

go/types, types2: Named.cleanup must also handle *Alias types

Named.cleanup is called at the end of type-checking to ensure that
a named type is fully set up; specifically that it's underlying
field is not (still) a Named type. Now it can also be an *Alias
type. Add this case to the respective type switch.

Fixes #68877.

Change-Id: I29bc0024ac9d8b0152a3d97c82dd28d09d5dbd66
Reviewed-on: https://go-review.googlesource.com/c/go/+/605977
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>