Firebaseへ匿名ユーザーのログイン
Authenticationのログイン方法はmail/password, Anonymousを有効にする

   
 @IBAction func loginOutClicked(_ sender: Any) {
        
        guard let user = Auth.auth().currentUser else { return }
        if user.isAnonymous {
            presentLoginController()
        } else {
            do {
                try Auth.auth().signOut()
                Auth.auth().signInAnonymously { (result, error) in
                    if let error = error {
                        debugPrint(error)
                    }
                     self.presentLoginController()
                }
            } catch {
                debugPrint(error)
            }
        }
    }