Add some const qualifiers enabled by typeof_unqual change on copyObject

Add some const qualifiers enabled by typeof_unqual change on copyObject

The recent commit to change copyObject() to use typeof_unqual allows
cleaning up some APIs to take advantage of this improved qualifier
handling.  EventTriggerCollectSimpleCommand() is a good example: It
takes a node tree and makes a copy that it keeps around for its
internal purposes, but it can't communicate via its function signature
that it promises not scribble on the passed node tree.  That is now
fixed.

Reviewed-by: David Geier <geidav.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/92f9750f-c7f6-42d8-9a4a-85a3cbe808f3%40eisentraut.org