Remove free_splay_tree cleanup

One spot in gdb uses a cleanup to free a splay tree.  This patch
introduces a unique_ptr specialization for this case.

ChangeLog
2017-10-09  Tom Tromey  <tom@tromey.com>

	* mi/mi-main.c (free_splay_tree): Remove.
	(list_available_thread_groups): Use splay_tree_up.
	* common/gdb_splay_tree.h: New file.

gdb/common/gdb_splay_tree.h

@@ -0,0 +1,42 @@
+/* GDB wrapper for splay trees.
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef GDB_SPLAY_TREE_H
+#define GDB_SPLAY_TREE_H
+
+#include "splay-tree.h"
+
+namespace gdb {
+
+struct splay_tree_deleter
+{
+  void operator() (splay_tree tree) const
+  {
+    splay_tree_delete (tree);
+  }
+};
+
+} /* namespace gdb */
+
+/* A unique pointer to a splay tree.  */
+
+typedef std::unique_ptr<splay_tree_s, gdb::splay_tree_deleter>
+    gdb_splay_tree_up;
+
+#endif /* ! GDB_SPLAY_TREE_H */