ir: Add some debugging facilities for the comparison machinery

ir: Add some debugging facilities for the comparison machinery

When looking at something else, I felt the need to write the
debugging facilities below to dump the content of the set of types
being compared, to better understand why some comparison take more or
less time.

	* src/abg-ir-priv.h: Include iostream to access std::cerr.
	(environment::priv::{dump_classes_being_compared,
	dump_fn_types_being_compared}): Define new member functions.
	* src/abg-ir.cc (dump_classes_being_compared)
	(dump_fn_types_being_compared): Define new functions.

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