Germany’s auto industry is sliding faster into decline, with weak demand, trade tensions and rising Chinese competition compounding the pressures on the country’s carmakers. Porsche on Thursday joined BMW and Mercedes-Benz in reporting weaker sales in China, where local competitors led by BYD Co. and Xiaomi Corp. are winning over buyers with competitively priced electric vehicles. BMW earlier this week cut its earnings outlook after demand in the world’s biggest auto market fell short of expectations. Mo ...