Address the -Wuse-after-free warning in ATExecAttachPartition()

In [1], Andres reported a -Wuse-after-free bug in the
ATExecAttachPartition() function. I've created a patch to address it
with pointers from Amit offlist.

The issue was that the partBoundConstraint variable was utilized after
the list_concat() function. This could potentially lead to accessing
the partBoundConstraint variable after its memory has been freed.

The issue was resolved by using the return value of the list_concat()
function, instead of using the list1 argument of list_concat(). I
copied the partBoundConstraint variable to a new variable named
partConstraint and used it for the previous references before invoking
get_proposed_default_constraint(). I confirmed that the
eval_const_expressions(), make_ands_explicit(),
map_partition_varattnos(), QueuePartitionConstraintValidation()
functions do not modify the memory location pointed to by the
partBoundConstraint variable. Therefore, it is safe to use it for the
next reference in get_proposed_default_constraint()

Attaching the patch. Please review and share the comments if any.
Thanks to Andres for spotting the bug and some off-list advice on how
to reproduce it.

[1]: https://www.postgresql.org/message-id/flat/202311151802.ngj2la66jwgi%40alvherre.pgsql#4fc5622772ba0244c1ad203f5fc56701

Best Regards,
Nitin Jadhav
Azure Database for PostgreSQL
Microsoft