[dev.typealias] cmd/compile: recognize type aliases but complain for now (not yet supported)

Added test file.

For #18130.

Change-Id: Ifcfd7cd1acf9dd6a2f4f3d85979d232bb6b8c6b1
Reviewed-on: https://go-review.googlesource.com/34988
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/nodes.go

@@ -74,6 +74,7 @@ type (
 	// Name Type
 	TypeDecl struct {
 		Name   *Name
+		Alias  bool
 		Type   Expr
 		Group  *Group // nil means not part of a group
 		Pragma Pragma