With 20 years of perfect sheath dresses behind him, the American designer more than deserved the Lifetime Achievement Award he received at this year’s CFDAs.   This season it’s about: Real clothes that are clean-lined and draped to flatter. Poplin dresses define New York-chic (and are just the thing for the office in summer), while airy-light cloque is understated and glamorous. Note the raw denim, which comes with a hint of stretch.