a common usage for sets is for tracking exactly one instance of a pointer
for a given period of time, after which the set's entries are purged and it
is reused
this macro enables the purge phase of such usage to reset the table to a
pristine state, avoiding future rehashing due to ballooning of deleted entries
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8498>