IO: match the signature for DispatchIO with dispatch_io_create

[compnerd]

The DispatchIO constructor maps to dispatch_io_create. The value on
Windows for the fd (which is actually a HANDLE) is being truncated
since dispatch_fd_t is actually Int not Int32 on Windows. Use the
dispatch_fd_t to ensure that the right size is always passed along.

[compnerd]

@swift-ci please test

[compnerd]

With this change, it is now possible to write an echo server for Windows:

import Dispatch
import ucrt
import WinSDK

let rx: DispatchQueue = DispatchQueue(label: "rx", qos: .userInitiated)

let hIn: HANDLE = GetStdHandle(STD_INPUT_HANDLE)
if hIn == INVALID_HANDLE_VALUE { exit(EXIT_FAILURE) }

let io: DispatchIO = DispatchIO(type: .stream, fileDescriptor: Int(bitPattern: hIn), queue: rx) { error in
  guard error == 0 else { exit(EXIT_FAILURE) }
}
io.setLimit(lowWater: 1)
io.setLimit(highWater: Int.max)

while true {
  io.read(offset: 0, length: Int.max, queue: rx) { done, data, errorCode in
    guard errorCode == 0 else {
      print("IO Error \(errorCode)");
      exit(EXIT_FAILURE)
    }

    guard let data = data else { return }
    data.withUnsafeBytes { (ptr: UnsafePointer<CChar>) -> () in
      print(">> \(String(cString: ptr))")
    }
  }
}

[drexin]

/cc @MadCoder Would you mind taking a look if you have a moment?

[gwynne]

Looks nice and straightforward to me.

[hixio-mh]

Pull request