This summer’s hit comedy “The Hangover” recently became the highest grossing R-rated comedy of all time. Considering the movie had a budget of $34 million and subsequently raked in over $247 million, you can bet director Todd Phillips (“Old School”) is making Warner Brothers do cartwheels and backflips into a giant pool of sweaty money. It’s no wonder Phillips new comedy, “Due Date,” starts shooting this fall. “Hangover” star Zach Galifianakis is already attached and Robert Downey Jr has just …