Fix clang_decls InstId for generic class export (#7633)

The exported class was being inserted with a type inst ID as the key
(and looked up that way elsewhere), but when checking if the generic
class was already exported, the `first_decl_id` was being used. Make it
consistent, and opt for `first_decl_id` everywhere since it provides a
better location for diagnostics.

toolchain/check/cpp/generate_ast.cpp

@@ -238,7 +238,7 @@ auto CarbonExternalASTSource::MapInstIdToClangDeclOrType(LookupResult lookup)
       } else if (auto generic_class =
                      context_->insts().TryGetAs<SemIR::GenericClassType>(
                          type_inst_id)) {
-        return ExportGenericClassToCpp(*context_, type_inst_id, *generic_class);
+        return ExportGenericClassToCpp(*context_, *generic_class);
       }
 
       return nullptr;