[Go] Fix overloading on an undefined type.

Examples/test-suite/go/overload_polymorphic_runme.go

@@ -6,6 +6,10 @@ func main(){
 	t := overload_polymorphic.NewDerived()
 	
 	if overload_polymorphic.Test(t) != 0 {
-		panic("failed")
+		panic("failed 1")
+	}
+
+	if overload_polymorphic.Test2(t) != 1 {
+		panic("failed 2")
 	}
 }

Examples/test-suite/overload_polymorphic.i

@@ -19,4 +19,8 @@ public:
 int test(Base* base){ return 0;}
 int test(int hello){ return 1; }
 
+class Unknown;
+int test2(Unknown* unknown) { return 0; }
+int test2(Base* base) { return 1; }
+
 %}