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 33-year-old crown prince has grand plans to diversify the kingdom’s oil-dependent economy. Those ambitions have been knocked off stride by foreign criticism of Saudi actions, including the murder of journalist Jamal Khashoggi. Foreign-direct investment, already in decline, remains anemic. …