David Bowie, the infinitely changeable, fiercely forward-looking songwriter who taught generations of musicians about the power of drama, images and personas, died on Sunday, two days after his 69th birthday. His death was confirmed by his publicist, Steve Martin, on Monday morning. No other details were provided. Mr. Bowie had been treated for cancer for the last 18 months, according to a statement on his social-media accounts. “David Bowie died peacefully today surrounded by his family,” a post on his Facebook page read.