Can't cast to a @:generic type parameter

> [this forum post](https://community.haxe.org/t/working-with-generics-entities-and-components/2331) got me thinking about this

This looks like it should work, since `C` is known at compile-time:

```haxe
@:generic function doCast(v:Any, c:Class):C {
return cast(v, C);
}
```

Currently, it fails with `Can't cast to a type parameter`, just like without `@:generic`.