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.