Linux: Use a DispatchSourceRead instead of a FileHandle.readabilityHandler
Linux: Use a DispatchSourceRead instead of a FileHandle.readabilityHandler

- swift-corelibs-foundation does not currently support FileHandle handlers
  so use DispatchSource.makeReadSource instead of
  FileHandle.readabilityHandlerForReading if not on macOS.

apple/swift-syntax,apple/swift-syntax,apple/swift-syntax