Otherwise, dropping the partitioned table will automatically drop
any previously-detached children, which would be unfortunate.

Ashutosh Bapat and Rahila Syed, reviewed by Amit Langote and by me.

Discussion: http://postgr.es/m/CAFjFpRdOwHuGj45i25iLQ4QituA0uH6RuLX1h5deD4KBZJ25yg@mail.gmail.com