“The Farewell” is a tender tale about family deceit. Five years ago, Lulu heard that her aunt, the mother of her brother, or her Nai Nai in Chinese, had stage-four lung cancer; the doctor said she had three months left to live. The family has never decided to tell her, but plans have been made to see her regularly. To explain the sudden arrival of every family member in Changchun, China, where Nai Nai lived, Lulu’s father came up with an innovative proposal.