cpp-common: add GDirUp

cpp-common: add GDirUp

Add GDirUP, a unique_ptr type to manage the lifetime of GDir objects.

Change-Id: I8814db0bec41918203af2f6d43a97a3e8d13c8c1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8294
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10845
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>