Fix creation of partition descriptor during concurrent detach

Fix creation of partition descriptor during concurrent detach

When a partition is being detached in concurrent mode, it is possible
for find_inheritance_children_extended() to return that partition in the
list, and immediately after that receive an invalidation message that
sets its relpartbound to NULL just before we read it.  (This can happen
because table_open() reads invalidation messages.)  Currently we raise
an error
  ERROR:  missing relpartbound for relation %u
about the situation, but that's bogus because the table is no longer a
partition, so we shouldn't be complaining about it.  A better reaction
is to retry the find_inheritance_children_extended call to get a new
list, which will no longer have the partition being detached.

Noticed while investigating bug #18377.

Backpatch to 14, where DETACH CONCURRENTLY appeared.

Discussion: https://postgr.es/m/202405201616.y4ht2qe5ihoy@alvherre.pgsql