Use gdb::function_view in iterate_over_live_ada_tasks

This changes iterate_over_live_ada_tasks to accept a
gdb::function_view.  This is needed by a subsequent patch.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ada-lang.h (ada_task_list_iterator_ftype): Now a
	gdb::function_view.
	(iterate_over_live_ada_tasks): Change type of argument.
	* ada-tasks.c (iterate_over_live_ada_tasks): Change type
	of argument.