gdb: add cmd_list_element::is_prefix

Same idea as the previous patch, but for prefix instead of alias.

gdb/ChangeLog:

	* cli/cli-decode.h (cmd_list_element) <is_prefix>: New, use it.

Change-Id: I76a9d2e82fc8d7429904424674d99ce6f9880e2b

gdb/cli/cli-decode.h

@@ -83,6 +83,10 @@ struct cmd_list_element
   bool is_alias () const
   { return this->alias_target != nullptr; }
 
+  /* Return true if this command is a prefix command.  */
+  bool is_prefix () const
+  { return this->subcommands != nullptr; }
+
   /* Points to next command in this list.  */
   struct cmd_list_element *next = nullptr;