Auto merge of #117213 - oli-obk:check_item_type_cleanup, r=estebank

 Reorder check_item_type diagnostics so they occur next to the corresponding `check_well_formed` diagnostics

The first commit is just a cleanup.

The second commit moves most checks from `check_mod_item_types` into `check_well_formed`, invoking the checks in lockstep per-item instead of iterating over all items twice.

Author: bors