hpb: refactor multibackend constants to hpb/multibackend.h

PiperOrigin-RevId: 758683214

Author: honglooker

hpb/hpb.h

@@ -16,17 +16,14 @@
 #include "google/protobuf/hpb/arena.h"
 #include "google/protobuf/hpb/extension.h"
 #include "google/protobuf/hpb/internal/template_help.h"
+#include "google/protobuf/hpb/multibackend.h"
 #include "google/protobuf/hpb/ptr.h"
 #include "google/protobuf/hpb/status.h"
 
-#define HPB_INTERNAL_BACKEND_UPB 1
-#define HPB_INTERNAL_BACKEND_CPP 2
-
 #if HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_UPB
 #include "google/protobuf/hpb/backend/upb/interop.h"
 #include "google/protobuf/hpb/backend/upb/upb.h"
 #include "upb/wire/decode.h"
-
 #elif HPB_INTERNAL_BACKEND == HPB_INTERNAL_BACKEND_CPP
 #include "google/protobuf/hpb/backend/cpp/cpp.h"
 #else

hpb/multibackend.h

@@ -0,0 +1,14 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2025 Google LLC.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+#ifndef GOOGLE_PROTOBUF_HPB_MULTIBACKEND_H__
+#define GOOGLE_PROTOBUF_HPB_MULTIBACKEND_H__
+
+#define HPB_INTERNAL_BACKEND_UPB 1
+#define HPB_INTERNAL_BACKEND_CPP 2
+
+#endif  // GOOGLE_PROTOBUF_HPB_MULTIBACKEND_H__