Sema: Force synthesized conformance to Differentiable in EmittedMembersRequest

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -2451,6 +2451,7 @@ EmittedMembersRequest::evaluate(Evaluator &evaluator,
   forceConformance(Context.getProtocol(KnownProtocolKind::Decodable));
   forceConformance(Context.getProtocol(KnownProtocolKind::Encodable));
   forceConformance(Context.getProtocol(KnownProtocolKind::Hashable));
+  forceConformance(Context.getProtocol(KnownProtocolKind::Differentiable));
 
   // The projected storage wrapper ($foo) might have dynamically-dispatched
   // accessors, so force them to be synthesized.