lib: rename bt_clock_class_origin_is_{unknown,known}

lib: rename bt_clock_class_origin_is_{unknown,known}

The "is unknown" getter leads to double negatives:

    if (!bt_clock_class_origin_is_unknown(cls))

... which is not great for readability.  Rename to
bt_clock_class_origin_is_known().

Change-Id: I27a9c9fec80b0e23851ee62fd2c897a1c695d2b1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13317
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>