Rollup merge of #134158 - compiler-errors:item-def-id, r=jackh726

Rename `projection_def_id` to `item_def_id`

Renames `projection_def_id` to `item_def_id`, since `item_def_id` is what we call the analogous method for ~~`AliasTerm`/`AliasTy`~~ `PolyExistentialProjection`. I keep forgetting that this one is not called `item_def_id`.