The play “Giant” by Mark Rosenblatt , now running at the Music Box Theater in New York, combines a big-name writer as its subject (Roald Dahl), a weighty topic (antisemitism), and a brilliant star in the leading role (John Lithgow). It’s directed by the great Nicholas Hytner. It can’t miss – and it doesn’t. The man, the topic, and the star turn by Lithgow are all enthralling.