ir: make 'debug(artefact)' support showing enums

ir: make 'debug(artefact)' support showing enums

While debugging something else, I realized that 'debug(artifact)'
couldn't show the enumerators of an enum. I also realized that we were
not showing the 'declaration-only-ness' of the artefact either.  This
patch fixes that.

	* src/abg-ir.cc (get_debug_representation): Add support for
	showing details for enums.  Also show declaration-only-ness for
	class or unions.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>