gienuminfo: Split out GIFlagsInfo as a derived type

Flag enums are already treated as a special kind of enum within
`GIInfoType`, so it would be tidier to give it its own `GType` too, with
a subtype relation to `GI_TYPE_ENUM_INFO`.

This will simplify implementing `GI_IS_ENUM_INFO` in a following commit
too.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: #3216

girepository/giflagsinfo.h

@@ -0,0 +1,46 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ * GObject introspection: Flags type
+ *
+ * Copyright 2024 GNOME Foundation, Inc.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#if !defined (__GIREPOSITORY_H_INSIDE__) && !defined (GI_COMPILATION)
+#error "Only <girepository.h> can be included directly."
+#endif
+
+#include <girepository/gitypes.h>
+
+G_BEGIN_DECLS
+
+#define GI_TYPE_FLAGS_INFO (gi_flags_info_get_type ())
+
+/**
+ * GI_IS_FLAGS_INFO:
+ * @info: an info structure
+ *
+ * Checks if @info is a [class@GIRepository.FlagsInfo] (or a derived type).
+ *
+ * Since: 2.80
+ */
+#define GI_IS_FLAGS_INFO(info) (G_TYPE_CHECK_INSTANCE_TYPE ((info), GI_TYPE_FLAGS_INFO))
+
+G_END_DECLS