No comment preservation for parenthesized expressions

@evmar ran into issues where generating a JSDoc comment for a type assertion for Closure did not get preserved.

Neither

``` ts
var x = (/** @type {Foo.Bar} */ y);
```

nor

``` ts
var x = /** @type {Foo.Bar} */ (y);
```

have their comments preserved, though I'm not sure which is more common.
