I'm having trouble performing bitwise operations with `CGImageAlphaInfo` and `CGBitmapInfo` in Swift.

In particular, I don't know how to port this Objective-C code:

```
bitmapInfo &= ~kCGBitmapAlphaInfoMask;
bitmapInfo |= kCGImageAlphaNoneSkipFirst;

```

The following straightforward Swift port produces the somewhat cryptic compiler error `'CGBitmapInfo' is not identical to 'Bool'` on the last line:

```
bitmapInfo &= ~CGBitmapInfo.AlphaInfoMask
bitmapInfo |= CGImageAlphaInfo.NoneSkipFirst

```

Looking at the source code I noticed that `CGBitmapInfo` is declared as a `RawOptionSetType` while `CGImageAlphaInfo` isn't. Maybe this has something to do with it? 

It doesn't help that the official documentation on bitwise operators doesn't cover enums.