message: mark as willreturn
  This was introduced in but not documented
  in LangRef
diff: @@ -928,6 +928,7 @@ public:
  case llvm::Intrinsic::sideeffect: {
  FnAttrs attrs;
  attrs.set(FnAttrs::InaccessibleMemOnly);
  + attrs.set(FnAttrs::WillReturn);
  attrs.set(FnAttrs::NoThrow);
  return make_unique<FnCall>(Type::voidTy, "", "#sideeffect", move(attrs));
  }
  