Consider parenthesized expressions with JSDoc to be a form of type assertions

To quote @weswigham on https://github.com/Microsoft/TypeScript/issues/4447#issuecomment-143099295:

> in [the] closure compiler you do inline casts with jsdoc comments like so:
> 
> ``` ts
> var x = /** @type {Foo.Bar} */ (y);
> ```

We should consider supporting this in Salsa.
