ctf: move ctf_visitor_generate_ir to header file

Move ctf_visitor_generate_ir to the `ast.h` header file, the header file
containing all the declarations for the CTF 1 metadata visitors.  This
will allow exposing it as a proper C++ class and remove the accessor
functions.

Change-Id: Ic13494d0040ee9e2214ecca45d8ea2e7b75ec4fa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8162
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12264
Tested-by: jenkins <jenkins@lttng.org>