[builtins] Migrate String.fromCharCode to TurboFan code stub.

test/mjsunit/regress/regress-string-from-char-code-tonumber.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.
+
+// Flags: --allow-natives-syntax
+
+var thrower = { [Symbol.toPrimitive]: function() { FAIL } };
+
+function testTrace(func) {
+  try {
+    func(thrower);
+    assertUnreachable();
+  } catch (e) {
+    assertTrue(e.stack.indexOf("fromCharCode") >= 0);
+  }
+}
+
+testTrace(String.fromCharCode);
+
+function foo(x) { return String.fromCharCode(x); }
+
+foo(1);
+foo(2);
+testTrace(foo);
+%OptimizeFunctionOnNextCall(foo);
+testTrace(foo);