ListVal method to clear the list to allow reusing w/o new construction

src/Type.h

@@ -341,6 +341,9 @@ public:
     void Append(TypePtr t);
     void AppendEvenIfNotPure(TypePtr t);
 
+    // Resets the list to be empty.
+    void Clear() { types.clear(); }
+
     detail::TraversalCode Traverse(detail::TraversalCallback* cb) const override;
 
 protected:

src/Val.h

@@ -684,6 +684,15 @@ public:
      */
     void Append(ValPtr v);
 
+    /**
+     * Empties the list.
+     * @param v  the value to append.
+     */
+    void Clear() {
+        vals.clear();
+        type->AsTypeList()->Clear();
+    }
+
     // Returns a Set representation of the list (which must be homogeneous).
     TableValPtr ToSetVal() const;