go/types, types2: coreType/String must consider Alias types

go/types, types2: coreType/String must consider Alias types

Fixes regression from Go 1.22.

For #67547.

Change-Id: Idd319b9d2a73c824caa2c821df0e2fcd4f58cb08
Reviewed-on: https://go-review.googlesource.com/c/go/+/587176
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>