Tesla Inc (TSLA.O) Chief Executive Officer Elon Musk has built a 9.2% stake in Twitter Inc (TWTR.N), a regulatory filing showed on Monday, potentially making him the biggest stakeholder in the micro-blogging site. Twitter shares soared 26% in premarket trading after the filing, which comes close on the heels of Musk tweeting that he was giving "serious thought" to building a new social ...