Warn when an interface is marked @final

Currently, we allow marking an interface as final, but then allow subtypes of it (eg, extending it or implementing it). We should warn that @final on interfaces is not supported.

``` js
/**
* @interface
* @final
*/
function Foo() {}

/**
* @interface
* @extends {Foo}
*/
function Bar() {}

/**
* @constructor
* @implements {Foo}
*/
function Baz() {}
```