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

**TypeScript Version:** nightly (2.5.0-dev.20170902) **Code** ```ts 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 ```ts 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 }; ```