Make ada_tasks_inferior_data::task_list an std::vector

Make ada_tasks_inferior_data::task_list an std::vector

This removes a VEC type.  It requires converting ada_tasks_inferior_data
to C++ (initializing fields, allocating with new).  It seems, however,
that the allocated ada_tasks_inferior_data structures are never freed
(that should be fixed separately).

gdb/ChangeLog:

	* ada-tasks.c (ada_task_info_s): Remove typedef.
	(DEF_VEC_O(ada_task_info_s)): Remove.
	(struct ada_tasks_inferior_data): Initialize fields.
	<task_list>: Make an std::vector.
	(get_ada_tasks_inferior_data): Allocate with new.
	(ada_get_task_number): Adjust.
	(get_task_number_from_id): Likewise.
	(valid_task_id): Likewise.
	(ada_get_task_info_from_ptid): Likewise.
	(iterate_over_live_ada_tasks): Likewise.
	(add_ada_task): Likewise.
	(read_known_tasks): Likewise.
	(ada_build_task_list): Likewise.
	(print_ada_task_info): Likewise.
	(info_task): Likewise.
	(task_command_1): Likewise.