[Ada] Adjust ada-tasks.c:ada_build_task_list

[Ada] Adjust ada-tasks.c:ada_build_task_list

Originally, this function had a parameter called `warn_if_null'
which would trigger a message to be printed on stdout if the
program was found to not use Ada tasking.  It used one of the printf_
functions for that, which is wrong when considering the context of
GDB/MI interpreters.

So, this patch changes this function to stop printing the message,
and leaves that part to the callers instead.  It also changes the
semantics slightly to return the number of tasks found, rather than
a yes/no answer.  Not strictly needed, but simple enough to do, and
potentially useful later.

gdb/ChangeLog:

        * ada-lang.h (ada_build_task_list): Remove parameter
        `warn_if_null'.
        * ada-tasks.c (ada_build_task_list): Remove parameter
        `warn_if_null'.  Adjust implementation and documentation.
        (valid_task_id, ada_get_environment_task)
        iterate_over_live_ada_tasks): Adjust call to ada_build_task_list.
        (info_tasks_command): Adjust implementation.
        (task_command): Likewise.
        * ravenscar-thread.c (ravenscar_find_new_threads): Fix call
        to ada_build_task_list.