exempt _Differentiation from verify_all_overlays test

Author: Marc Rasi

validation-test/ParseableInterface/verify_all_overlays.py

@@ -49,7 +49,7 @@
     else:
         continue
 
-    if module_name == "Swift" or module_name == "SwiftLang":
+    if module_name in ["_Differentiation", "Swift", "SwiftLang"]:
         continue
 
     # swift -build-module-from-parseable-interface