Issue Description:

Since the stdlib pointer types started representing null pointers as Optional.none, it has become difficult to pass a null pointer as a CVarArg.

Here is a working example from Stack Overflow user MartinR: https://stackoverflow.com/a/32064355

let nullPtr = unsafeBitCast(0, to: OpaquePointer.self)
let otherTitles : [CVarArg] = otherButtonTitles + [nullPtr]
getVaList(...)/withVaList(...)

Having to use unsafeBitCast is awkward; it would be better if a null pointer could be used directly in this context. Perhaps the right solution is something like extension Optional: CVarArg where Wrapped == OpaquePointer/UnsafePointer/.... (For that matter, it would also be nice to have an easier way to obtain a null OpaquePointer, perhaps something like init?() { return nil }.)