Auto merge of #143357 - cjgillot:no-assoc-item-kind, r=compiler-errors

Retire hir::*ItemRef.

This information was kept for various places that iterate on HIR to know about trait-items and impl-items.

This PR replaces them by uses of the `associated_items` query that contain pretty much the same information.

This shortens many spans to just `def_span`, which can be easier to read.

Author: bors