<p>LAS VEGAS — Another billionaire is angering environmentalists these days: Democratic hero Warren Buffett. </p><p>For Democrats, the world’s third-richest person is a champion who has advocated higher taxes for millionaires and donated money to put Hillary Clinton in the White House. But green-energy advocates are expressing increasing frustration with Buffett, whose sprawling business empire is opposing them on fronts including solar power, oil train safety and the Keystone XL oil pipeline.