cmd/compile: support generic alias type

Type parameters on aliases are now allowed after #46477 accepted.

Updates #46477
Fixes #68054

Change-Id: Ic2e3b6f960a898163f47666e3a6bfe43b8cc22e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/593715


Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>