Cast T[] to integral type: deprecation -> error.

Having a look at: http://dlang.org/deprecate.html

I see that casting from T[] to an integral type became an error in 2.061.  However it still produces the deprecation warning in the frontend before finally being rejected by the backend.

```
Deprecation: casting int[] to ulong is deprecated
Error: e2ir: cannot cast [1, 2] of type int[] to type ulong
```

This change makes it correctly error in the frontend, and probably puts the final nail in the coffin to make it error -> gone.