Fifteen or 20 years ago, the singer Josh Groban would have seemed well suited to perform in a production of “Sweeney Todd: The Demon Barber of Fleet Street” — but not in the title role. With his robust, limpid bari-tenor and mild, affable persona, Mr. Groban might have been a candidate for the role of Anthony, the male ingénue who rescues Sweeney while the latter is escaping from prison, then falls in love with his teenage daughter.