Add support for custom layout guide instances in interface builder by using custom UIView subclass. Uses method swizzling to swap out the view during nib decoding.

SpacerView+LayoutGuide.swift

private extension UIView {
    
    private struct Static {
        static let initialize: (AnyClass) -> Void = { (aClass: AnyClass!) in
            let swizzle: (Selector, Selector) -> Void = { (original, swizzled) in
                let originalSelector = original
                let swizzledSelector = swizzled
                
                let originalMethod = class_getInstanceMethod(aClass, originalSelector)
                let swizzledMethod = class_getInstanceMethod(aClass, swizzledSelector)
                
                let didAddMethod = class_addMethod(aClass, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod))
                
                if didAddMethod {
                    class_replaceMethod(aClass, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod))
                } else {
                    method_exchangeImplementations(originalMethod, swizzledMethod)
                }
            }
            
            let pairs = [
                #selector(UIView.addSubview(_:)): #selector(UIView.layoutGuide_addSubview(_:))
            ]
            
            pairs.forEach { (key, value) in
                swizzle(key, value)
            }
        }
    }
    
    open override class func initialize() {
        // make sure this isn't a subclass
        if self !== UIView.self {
            return
        }
        
        Static.initialize(self)
    }
    
    private dynamic func layoutGuide_addSubview(_ view: AnyObject!) {
        if let layoutGuide = view as? UILayoutGuide {
            addLayoutGuide(layoutGuide)
            return
        }
        
        layoutGuide_addSubview(view)
    }
}

class SpacerView: UIView {
    
    @IBInspectable var identifier: String?
    
    override func awakeAfter(using aDecoder: NSCoder) -> Any? {
        return UILayoutGuide(coder: aDecoder)
    }
}