The exact same thing that happens now: someone dies, usually selected arbitrarily based upon the driver's selfish desire to minimize harm to themselves. Because

The exact same thing that happens now: someone dies, usually selected arbitrarily based upon the driver's selfish desire to minimize harm to themselves. Because that situation doesn't allow enough time to make anything except instinctual choices. The driver is found at fault for that death if they were not driving legally. Otherwise, and very often times regardless, the whole thing is written off as a tragic accident (millions happen every year, just in the USA), and the insurance people do their thing. Ethical dilemmas are a non-problem for self-driving cars.