stdlib: restructure for OS family layout of SDK overlay

The SDK directory is now confusing as the Windows target also has a SDK
overlay.  In order to make this more uniform, move the SDK directory to
Darwin which covers the fact that this covers the XNU family of OSes.
The Windows directory contains the SDK overlay for the Windows target.

Author: compnerd

Darwin/Foundation-swiftoverlay/NSStringAPI.swift

@@ -19,7 +19,7 @@
 //
 // This file is shared between two projects:
 //
-// 1. https://github.com/apple/swift/tree/master/stdlib/public/SDK/Foundation
+// 1. https://github.com/apple/swift/tree/master/stdlib/public/Darwin/Foundation
 // 2. https://github.com/apple/swift-corelibs-foundation/tree/master/Foundation
 //
 // If you change this file, you must update it in both places.
@@ -2176,4 +2176,4 @@ extension StringProtocol {
   ) throws {
     fatalError("unavailable function can't be called")
   }
-}
+}