clean up TypeDefinitionConverter code

AssetStudioUtility/Unity.CecilTools/ElementType.cs

@@ -0,0 +1,21 @@
+// Unity C# reference source
+// Copyright (c) Unity Technologies. For terms of use, see
+// https://unity3d.com/legal/licenses/Unity_Reference_Only_License
+
+using System;
+using Mono.Cecil;
+
+namespace Unity.CecilTools
+{
+    static public class ElementType
+    {
+        public static TypeReference For(TypeReference byRefType)
+        {
+            var refType = byRefType as TypeSpecification;
+            if (refType != null)
+                return refType.ElementType;
+
+            throw new ArgumentException(string.Format("TypeReference isn't a TypeSpecification {0} ", byRefType));
+        }
+    }
+}