[Interpreter] Add ForInPrepare runtime function which returns a ObjectTriple.

src/runtime/runtime-utils.h

Index: src/runtime/runtime-utils.h
diff --git a/src/runtime/runtime-utils.h b/src/runtime/runtime-utils.h
index ded2c090c88ba9f2a908732c75850e3e71d8a2c0..c673b5a1557d45d1cf9ee65b1728f113d0179b34 100644
--- a/src/runtime/runtime-utils.h
+++ b/src/runtime/runtime-utils.h
@@ -162,6 +162,22 @@ static inline ObjectPair MakePair(Object* x, Object* y) {
 }
 #endif
 
+
+// A mechanism to return a triple of Object pointers. In all calling
+// conventions, a struct of two pointers is returned in memory,
+// allocated by the caller, and passed as a pointer in a hidden first parameter.
+struct ObjectTriple {
+  Object* x;
+  Object* y;
+  Object* z;
+};
+
+static inline ObjectTriple MakeTriple(Object* x, Object* y, Object* z) {
+  ObjectTriple result = {x, y, z};
+  // ObjectTriple is assigned to a hidden first argument.
+  return result;
+}
+
 }  // namespace internal
 }  // namespace v8