Allow product discount functions to be combined with order coupon discounts

[hagsonz]

Currently we have a major problem preventing us from combining a function generated product discount with a native order scoped coupon code.

we have the next scenario:

1. display products as an offer in checkout
2. customer to adds one of the products and receives 15% discount on that line
3. customer inserting a coupon which applies to all order

result:
the function generated product discount disappears and the coupon applies to all order.
if the customer will remove the coupon, the product discount will appear again.

desired result
order discount will apply to the whole amount including the discounted product (as it is now in Shopify Scripts).

In overall, this limitation prevents us from providing a Buy X get Y (for free or for some discount) - "Order Bump" and "Gift with purchase" discounts.
furthermore, some loyalty apps (e.g Yotpo) are generating coupons - this means a customer can't get loyalty discount with a custom discount created by us.