cmd/compile: add missing Unalias call when writing type alias

cmd/compile: add missing Unalias call when writing type alias

Fixes #65778

Change-Id: I93af42967c7976d63b4f460b7ffbcb9a9c05ffe7
Reviewed-on: https://go-review.googlesource.com/c/go/+/565995
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>