The correct answer is here:
https://stackoverflow.com/a/43110590/566360

1. Embed the UILabel inside a UIView (Editor -> Embed In -> View)
2. Use constraints to fit the UILabel to the UIView (for example, trailing space, top space, and leading space to superview constraints)

The UIStackView will stretch out the UIView to fit properly, and the UIView will constrain the UILabel to multiple lines.