Error on non-constant parameters to a type. (#4215)

At present, this is a crash bug. I don't know whether this is the best
fix, but I figure it'll work until zygoloid has a chance to look.

toolchain/check/handle_class.cpp

@@ -224,6 +224,9 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
        .self_type_id = SemIR::TypeId::Invalid,
        .inheritance_kind = inheritance_kind}};
 
+  RequireGenericParams(context, class_info.implicit_param_refs_id);
+  RequireGenericParams(context, class_info.param_refs_id);
+
   MergeOrAddName(context, node_id, name_context, class_decl_id, class_decl,
                  class_info, is_definition, is_extern,
                  introducer.modifier_set.GetAccessKind());