Fix intra-query memory leakage in nodeProjectSet.c.

Both ExecMakeFunctionResultSet() and evaluation of simple expressions
need to be done in the per-tuple memory context, not per-query, else
we leak data until end of query.  This is a consideration that was
missed while refactoring code in the ProjectSet patch (note that in
pre-v10, ExecMakeFunctionResult is called in the per-tuple context).

Per bug #14843 from Ben M.  Diagnosed independently by Andres and myself.

Discussion: https://postgr.es/m/20171005230321.28561.15927@wrigleys.postgresql.org

src/backend/executor/execSRF.c

@@ -467,6 +467,8 @@ ExecInitFunctionResultSet(Expr *expr,
  * function itself.  The argument expressions may not contain set-returning
  * functions (the planner is supposed to have separated evaluation for those).
  *
+ * This should be called in a short-lived (per-tuple) context.
+ *
  * This is used by nodeProjectSet.c.
  */
 Datum