Force -swift-version 3 for Standard Library and overlays.

Author: tkremenek

cmake/modules/SwiftSource.cmake

@@ -268,6 +268,11 @@ function(_compile_swift_files
     endif()
   endif()
 
+  # Force swift 3 compatibility mode for Standard Library and overlay.
+  if (SWIFTFILE_IS_STDLIB_CORE OR SWIFTFILE_IS_SDK_OVERLAY)
+    list(APPEND swift_flags "-swift-version" "3")
+  endif()
+
   if(SWIFTFILE_IS_SDK_OVERLAY)
     list(APPEND swift_flags "-autolink-force-load")
   endif()