Allow class inheritance in ambient contexts, even with a base private constructor

首先，使用所引用的 TypeScript nightly 行为编译 issue 中的 ambient namespace 示例。跟踪扩展 Foo.Bar 时 checker 给出的诊断，并将其与预期接受该声明的情况进行比较。完成的标准是示例能够编译，同时构造和对象字面量赋值仍然报错。

TypeScript Version: nightly (2.5.0-dev.20170902)

Code

declare namespace Foo {
    class Bar {
        private constructor();
        Box: string;
    }
    class Baz extends Bar {
        private constructor();
    }
    function generator(): Baz;
}

let x = Foo.generator();
x.Box = 'abcd';

Expected behavior:
Compile without error.

Actual behavior:
error TS2675: Cannot extend a class 'Foo.Bar'. Class constructor is marked as private.

Use case
This would allow declaring the types of host-supplied objects, with the following features:

1. can inherit members from each other
2. cannot be inherited from using standard Typescript/Javascript classes
3. cannot be assigned to from an object literal with matching members
4. cannot be constructed, only returned from a factory method

declare namespace com.sun.star.text {
    class XTextTablesSupplier {
        private constructor();
        private typekey: XTextTablesSupplier;
        readonly TextTables: any;
    }
    class GenericTextDocument extends XTextTablesSupplier {
        private constructor();
        private typekey1: GenericTextDocument;
        CharacterCount: number;
    }
}
declare function createInstance(typename: 'com.sun.star.text.GenericTextDocument'): com.sun.star.text.GenericTextDocument;

let x = createInstance('com.sun.star.text.GenericTextDocument'); // OK

x = new com.sun.star.text.GenericTextDocument(); // Error

let y = x.TextTables; // OK

// The following would be an error
x = {
    TextTables: '',
    CharacterCount: 5
};