Properly set function index in CallSite constructor

test/mjsunit/callsite.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.
-
-Error.prepareStackTrace = (e,s) => s;
-var constructor = Error().stack[0].constructor;
-
-// Second argument needs to be a function.
-assertThrows(()=>constructor({}, {}, 1, false), TypeError);
-
-var receiver = {};
-function f() {}
-
-var site = constructor.call(null, receiver, f, {valueOf() { return 0 }}, false);
-assertEquals(receiver, site.getThis());
-assertEquals(1, site.getLineNumber());
-assertEquals(1, site.getColumnNumber());