Set partitioned_rels appropriately when UNION ALL is used.

In most cases, this omission won't matter, because the appropriate
locks will have been acquired during parse/plan or by AcquireExecutorLocks.
But it's a bug all the same.

Report by Ashutosh Bapat.  Patch by me, reviewed by Amit Langote.

Discussion: http://postgr.es/m/CAFjFpRdHb_ZnoDTuBXqrudWXh3H1ibLkr6nHsCFT96fSK4DXtA@mail.gmail.com