Cleanup class_decl inifite comparison detection
    
    We recently used a new type interned_string_set_type to detect
    infinite comparison of function_type type.  This patch uses the same
    type for class_decl infinite comparison detection.
    
    	* src/abg-ir.cc (environment::priv::classes_being_compared_): Make
    	this use the new interned_string_set_type type.
    	(class_decl::priv::{mark_as_being_compared, comparison_started}):
    	Adjust.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>