Accept base classes which types are compatible with class type

Accept base classes which types are compatible with class type

Until now, a base class had to be a class itself.  It couldn't be a
typedef to a class, for instance. Clang's debug info does allow base
classes which are compatible with classes (e.g, typedefs of classes),
which is correct.  We ought to accept that.  Hence this patch.

	* include/abg-fwd.h (is_compatible_with_class_type): Declare a new
	overload.
	* src/abg-dwarf-reader.cc (build_class_type_and_add_to_ir): Rather
	than requiring that base classes be of class type, just require
	that they be compatible with class types.
	* src/abg-ir.cc (is_compatible_with_class_type): Define a new
	overload.

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