During his surprise appearance at the World Economic Forum in Davos, Elon Musk mused about reversing aging, enlisting humanoid robots to look after kids and launching solar-powered data centers into space — that last one, he said, was a “no-brainer.”Less obvious was how all these endeavors will be funded. Already, the staggering cost of developing artificial intelligence, then putting it to work in furtherance of Musk’s manifold visions, is beginning to bend the structure of his sprawling empire...