The feel-good La La Land, director Damien Chazelle's prohibitive awards favorite, is a movie of mystery. Can Emma Stone sing? (She can.) Can Ryan Gosling sing and dance? (As a singer, he makes an OK dancer.) The real and lasting question rising from its smoggy, sunny success, however, is this one: Why don’t they make more musicals? Every week a pigeon flies in from “The Death of Cinema”-land with a horror story about