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.

stdlib/public/CMakeLists.txt

@@ -68,13 +68,12 @@ if(SWIFT_BUILD_SDK_OVERLAY)
   add_subdirectory(Platform)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
-  if(SWIFT_BUILD_SDK_OVERLAY)
-    add_subdirectory(SDK)
-  endif()
-endif()
-
 if(SWIFT_BUILD_SDK_OVERLAY)
+  list_intersect("${SWIFT_APPLE_PLATFORMS}" "${SWIFT_SDKS}" building_darwin_sdks)
+  if(building_darwin_sdks)
+    add_subdirectory(Darwin)
+  endif()
+
   if(WINDOWS IN_LIST SWIFT_SDKS)
     add_subdirectory(Windows)
   endif()