Fix UNION planner datatype issue

Fix UNION planner datatype issue

66c0185a3 gave the planner the ability to have union child queries
provide the union planner with pre-sorted input so that UNION queries
could be more efficiently implemented using Merge Append.

That commit overlooked checking that the UNION target list and the union
child target list's types all match.  In some corner cases, this could
result in the planner producing sorts using the sort operator of the
top-level UNION's target list type rather than of the union child's
target list's type.  The implications of this range from silently
working correctly, despite using the wrong sort operator all the way up
to a segmentation fault.

Here we fix by adjusting the planner so it makes no attempt to have the
subquery produce pre-sorted results when the data type of the UNION
target list and the types from the subquery target list don't match
exactly.

Backpatch to 17, where 66c0185a3 was introduced.

Reported-by: Jason Smith <dqetool@126.com>
Diagnosed-by: Tom Lane <tgl@sss.pgh.pa.us>
Bug: 18764
Discussion: https://postgr.es/m/18764-63ad667ea26e877a%40postgresql.org
Backpatch-through: 17