title: addSubview and addArrangedSubview not showing content
body: <p>I'm adding a <code>UILabel</code> to a <code>UIView</code> and then adding the <code>UIView</code> to a <code>UIStackView</code>, but the UILabel is not showing.</p>
  <pre class="lang-swift prettyprint-override"><code>override func viewDidLoad() {
      super.viewDidLoad()
      
      configureStackView()
      
      label.text = &quot;test&quot;
      containerView.addSubview(label)
      stackView.addArrangedSubview(containerView)
      view.addSubview(stackView)
      
      setStackViewConstraints()
  }
  
  func configureStackView() {
      stackView.axis = .vertical
      stackView.distribution = .fill
      stackView.alignment = .fill
      stackView.spacing = 5
  }
  
  func setStackViewConstraints() {
       stackView.translatesAutoresizingMaskIntoConstraints = false
       stackView.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
       stackView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
       stackView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
       stackView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
       stackView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
   }
  </code></pre>
  <p>I tried setting some sort of size for the view, but doesn't work:</p>
  <pre class="lang-swift prettyprint-override"><code>containerView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
  </code></pre>
  <p>or setting constraints to the label to the <code>containerView</code> for <code>widthAnchor</code>, <code>leadingAnchor</code>, or <code>trailingAnchor</code>, but that doesn't work either.</p>