[es5] call ToString() on argument in String.prototype.concat() fast case

test/mjsunit/string-concat.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+function Stringified(toString) {
+  var valueOf = "-" + toString + "-";
+  return {
+    toString: function() { return toString; },
+    valueOf: function() { return valueOf; }
+  };
+}
+
+assertEquals("a.b.", "a.".concat(Stringified("b.")));
+assertEquals("a.b.c.", "a.".concat(Stringified("b."), Stringified("c.")));