lib: add enum bt_resolve_field_xref_status

lib: add enum bt_resolve_field_xref_status

Add the bt_resolve_field_xref_status enumeration, and use it throughout
the resolve-field-path.c file to make error handling more explicit, and
clearer, in my opinion.

Change-Id: I51ddb22ac41dde0f58ca2e5d7fb90267984412e9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7304
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>