[Ada] Add field "thread-id" in -ada-task-info output

[Ada] Add field "thread-id" in -ada-task-info output

This patch enhances the -ada-task-info command, adding a new
field called "thread-id" in the command output.  This field
provides the ID of the task's corresponding thread (if that
information could not be determined, the field is omitted).

This field is useful in the context of GDB/MI users, because
it allows a quick translation from Ada task to thread, and
thus easy use of all the commands that take a --thread argument.

gdb/ChangeLog:

        * ada-tasks.c (print_ada_task_info): Add "thread-id" field
        in output of -ada-task-info GDB/MI command.