cmd/compile/internal/types2: export Named.Orig as Named.Origin

This is a slightly adjusted port of CL 350996 from go/types to types2.

Change-Id: I8f5902ecb20a4b4d2a5ef0d3641d857bc3a618e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/351170
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/noder/writer.go

@@ -296,7 +296,7 @@ func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
 		// TODO(mdempsky): Why do we need to loop here?
 		orig := typ
 		for orig.TypeArgs() != nil {
-			orig = orig.Orig()
+			orig = orig.Origin()
 		}
 
 		w.code(typeNamed)