When diagnosing a duplicated name, add the name to the diagnosis (#4902)

In order to have the name available for diagnostics, we now always set
`NameId` in `NameContext` and put `poisoning_loc_id` as part of the
union with `resolved_inst_id` instead (since we never need both).

toolchain/check/handle_interface.cpp

@@ -105,7 +105,8 @@ static auto BuildInterfaceDecl(Context& context,
       }
     } else {
       // This is a redeclaration of something other than a interface.
-      DiagnoseDuplicateName(context, name_context.loc_id, existing_id);
+      DiagnoseDuplicateName(context, name_context.name_id, name_context.loc_id,
+                            existing_id);
     }
   }