Convert ada-tasks.c to type-safe registry API

Convert ada-tasks.c to type-safe registry API

This changes ada-tasks.c to use the type-safe registry API.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* ada-tasks.c (ada_tasks_pspace_data_handle): Change type.
	(get_ada_tasks_pspace_data): Update.
	(ada_tasks_pspace_data_cleanup): Remove.
	(_initialize_tasks): Update.
	(ada_tasks_inferior_data_handle): Change type.
	(get_ada_tasks_inferior_data): Update.
	(ada_tasks_inferior_data_cleanup): Remove.
	(struct ada_tasks_pspace_data): Add initializers.