ctf: use unique_ptr to manage ctf_visitor_generate_ir lifetime

Define ctf_visitor_generate_ir::UP as a unique_ptr type with a custom
deleter that calls ctf_visitor_generate_ir_destroy.  Make
ctf_visitor_generate_ir_create return a ctf_visitor_generate_ir::UP and
adjust relevant callers / callees.

Change-Id: I9db22bd66e69473e67c2571e0b2bb7682863cff7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8163
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12265
Tested-by: jenkins <jenkins@lttng.org>