Move name field from AssocItem to AssocKind variants.

To accurately reflect that RPITIT assoc items don't have a name. This
avoids the use of `kw::Empty` to mean "no name", which is error prone.

Helps with #137978.

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -32,7 +32,7 @@ pub(super) fn find_opaque_ty_constraints_for_impl_trait_in_assoc_type(
     for &assoc_id in tcx.associated_item_def_ids(impl_def_id) {
         let assoc = tcx.associated_item(assoc_id);
         match assoc.kind {
-            ty::AssocKind::Const | ty::AssocKind::Fn { .. } => {
+            ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. } => {
                 locator.check(assoc_id.expect_local())
             }
             // Associated types don't have bodies, so they can't constrain hidden types