By Daniel Kline Taco Bell used to say “think outside the bun,” McDonald’s has taken a different approach. Yum! Brands (YUM) – Get Yum! Brands Inc. Report Taco Bell used to use the tagline “think outside the bun” to highlight how its Mexican menu differed from the sandwiches offered by McDonald’s (MCD) – Get McDonald’s Corporation Report, Wendy’s (WEN) – Get Wendy’s Company (The) Report, and Restaurant Brands International’s (QSR) – Get Restaurant Brands International Inc. Report Burger King. The fast-food chain wanted customers to embrace the idea that they could get something different at Tac…