Minor Ada task cleanups

While working on the Ada task code, I noticed a few things that could
be cleaned up:

* task_list_valid_p was not set in all cases in ada_build_task_list.
  This causes many needless re-fetches of the task list.

* task_list_valid_p can be bool, and various functions can also return
  bool.

* Nothing checks the return value of read_known_tasks, so it can be
  changed to return void.

* The call to ada_build_task_list in
  ravenscar_thread_target::update_thread_list is redundant, because
  this is the first thing done by iterate_over_live_ada_tasks.

Tested using the internal AdaCore test suite against a ravenscar
target.

gdb/ChangeLog
2019-02-19  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c
	(ravenscar_thread_target::update_thread_list): Don't call
	ada_build_task_list.
	* ada-lang.h (ada_build_task_list): Don't declare.
	* ada-tasks.c (struct ada_tasks_inferior_data)
	<task_list_valid_p>: Now bool.
	(read_known_tasks, ada_task_list_changed)
	(ada_tasks_invalidate_inferior_data): Update.
	(read_known_tasks_array): Return bool.
	(read_known_tasks_list): Likewise.
	(read_known_tasks): Return void.
	(ada_build_task_list): Now static.

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