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_class.cpp

@@ -164,7 +164,8 @@ static auto MergeOrAddName(Context& context, Parse::AnyClassDeclId node_id,
 
   if (!prev_class_id.has_value()) {
     // This is a redeclaration of something other than a class.
-    DiagnoseDuplicateName(context, name_context.loc_id, prev_id);
+    DiagnoseDuplicateName(context, name_context.name_id, name_context.loc_id,
+                          prev_id);
     return;
   }