Fix T81077 id_management test on macOS

This looks like a optimizer bug where it makes wrong assumptions.
The code inside lib_id_delete:264 on rBafd13710b897cc1c11b
`for (id = last_remapped_id->next; id; id = id->next) {..}`
is not executed in release/relwithdebinfo builds.

This can be "fixed" by several ways:
- Adding a line that prints the `last_remapped_id->name` right before
  the said for-loop starts.
- Turning off optimization for the whole function `id_delete`:
  `#pragma clang optimize off/on` Ray Molenkamp
- Marking `last_remapped_id` volatile. Julian Eisel
- Marking `tagged_deleted_ids` volatile. But it adds a warning when
  calling `BLI_addtail`: discards volatile qualifier. Discovered by
  accident.

Fix T81077

Reviewed By: mont29

Maniphest Tasks: T81077

Differential Revision: https://developer.blender.org/D9315

source/blender/blenkernel/intern/lib_id_delete.c

@@ -261,7 +261,9 @@ static void id_delete(Main *bmain, const bool do_tagged_deletion)
     bool keep_looping = true;
     while (keep_looping) {
       ID *id, *id_next;
-      ID *last_remapped_id = tagged_deleted_ids.last;
+      /* Marked volatile to avoid a macOS Clang optimization bug. See T81077.
+       * #last_remapped_id.next is assumed to be NULL by optimizer which is wrong. */
+      volatile ID *last_remapped_id = tagged_deleted_ids.last;
       keep_looping = false;
 
       /* First tag and remove from Main all datablocks directly from target lib.