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/impl_wf_check.rs

@@ -119,7 +119,7 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
                         vec![]
                     }
                 }
-                ty::AssocKind::Fn { .. } | ty::AssocKind::Const => vec![],
+                ty::AssocKind::Fn { .. } | ty::AssocKind::Const { .. } => vec![],
             }
         })
         .collect();