Saudi Arabia’s national oil company has agreed to buy a 70% stake in Saudi Basic Industries Corp., the kingdom’s petrochemicals firm, for $69.1 billion, giving Crown Prince Mohammed bin Salman’s agenda a massive jolt of cash. The merger of Aramco and Sabic, as the petrochemicals firm is known, would join the country’s two largest companies. In doing so, the deal will hand the Saudi sovereign-wealth fund roughly the same amount of money it had expected to reap from the now-postponed Aramco initial public offering. …