message/pipeline: add two Unalias calls

Change-Id: I692c027e2b97ee5509a63a2c88265759fc766800
Reviewed-on: https://go-review.googlesource.com/c/text/+/645016
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

message/pipeline/extract.go

@@ -148,13 +148,13 @@ func (x *extracter) processGlobalVars() {
 				}
 				t := a.Type()
 				for {
-					p, ok := t.(*types.Pointer)
+					p, ok := types.Unalias(t).(*types.Pointer)
 					if !ok {
 						break
 					}
 					t = p.Elem()
 				}
-				if b, ok := t.(*types.Basic); !ok || b.Kind() != types.String {
+				if b, ok := types.Unalias(t).(*types.Basic); !ok || b.Kind() != types.String {
 					continue
 				}
 				x.visitInit(a, s.Val)