SelectableEventLoop: make Darwin the special case (#1449)
SelectableEventLoop: make Darwin the special case (#1449)

Treat the `autoreleasepool` case for Darwin as the special case rather
than Linux as the special case.  This is following the case throughout
the rest of the code base.  This is also needed for the Windows port.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio