Object.prototype.toString must reflect mutated @@toStringTag values for primitives

test/mjsunit/regress/regress-5780.js

+// Copyright 2016 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 testMutatedPrimitiveToStringTag(primitive) {
+  Object.defineProperty(
+    primitive.__proto__, Symbol.toStringTag,
+    {value: "bogus", configurable: true, writable: false, enumerable: false});
+  assertEquals("[object bogus]", Object.prototype.toString.call(primitive));
+}
+
+testMutatedPrimitiveToStringTag('');
+testMutatedPrimitiveToStringTag(true);
+testMutatedPrimitiveToStringTag(42);
+testMutatedPrimitiveToStringTag(42.42);
+testMutatedPrimitiveToStringTag(Symbol());