Note: The full credit should go to null and akash. They contributed by answering half of the question each.
You can't have one of the icons from Apple that are avaible directly in Interface Builder and have a custom title. Instead, you must download the icon you want to use (Apple-looking icons are avaible at icons8), add it to the project resources and then pick it from the dropdown menu at Inspector -> Attributes inspector -> Bar Item -> Image.