[SR-1941] Dispatch object creation can return nil despite non-nil type annotations

Issue Description:

Attempting to create a dispatch source or I/O channel is a fallible operation. In Swift 2.2 the functions used for creating them are annotated as returning implicitly-unwrapped optionals, and nil is returned when creation fails. This can be checked for and handled appropriately.

In Swift 2.3 and 3.0 the functions/initializers are annotated as returning non-nil values, but nil can still be returned. This can be reproduced by providing invalid parameters.

The code below will run successfully under Swift 2.2, but will crash at runtime under 2.3 and 3.0 despite the lack of any forced or implicit unwrapping.

#if swift(>=3.0)
    
    let channel = DispatchIO(type: .stream, path: "../bad.path", oflag: O_RDONLY, mode: S_IRUSR, queue: .main) { (error) in
        print(error)
    }
    print(channel) // Crash
    
    let source = DispatchSource.signal(signal: Int32.min)
    print(source) // Crash
    
#elseif swift(>=2.3)
    
    let channel = dispatch_io_create_with_path(DISPATCH_IO_STREAM, "../bad.path", O_RDONLY, S_IRUSR, dispatch_get_main_queue()) { (error) in
        print(error)
    }
    print(channel) // Crash
    
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, UInt.max, 0, dispatch_get_main_queue())
    print(source) // Crash
    
#else
    
    let channel = dispatch_io_create_with_path(DISPATCH_IO_STREAM, "../bad.path", O_RDONLY, S_IRUSR, dispatch_get_main_queue()) { (error) in
        print(error)
    }
    print(channel ?? "Nil") // Nil
    
    let source = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, UInt.max, 0, dispatch_get_main_queue())
    print(source ?? "Nil") // Nil
    
#endif

Start by reproducing the failure with the DispatchIO initializer and DispatchSource.signal, or the C entry points dispatch_io_create_with_path and dispatch_source_create, using the invalid parameters shown. Compare the Swift 2.2 nil behavior with Swift 2.3 and 3.0, then trace how the non-nil annotations are exposed; done should mean invalid creation cannot crash callers that handle the result as described.