Noah Baumbach, whose new film "While We're Young" opens this weekend, has become the master of a genre unto himself: the personal counterfactual. In "Greenberg" and "Frances Ha," and now once again, he looks to his own experience and character, and then films dramas that walk down the road not taken or that leave himself out of a story that involves him. "While We're Young" is an imaginative fiction about a filmmaker…
