Try harder to handle pointer/reference to void

Try harder to handle pointer/reference to void

When a pointer_type_def or reference_type_def is initialized with an
empty pointed-to type, we want consider that as a pointer/reference to
void.

This patch does that.  That helps to avoids to later crash because the
pointed-to-type is empty.  Also, this patch fixes spots where the
pointed-to-type seems nonetheless empty.

I have used the two different versions of libstdc++ from RHEL 6.5 and RHEL
7 to spot and fix these issues.

	* include/abg-fwd.h (type_or_void): Declare new function.
	* src/abg-ir.cc (type_or_void): Define it.
	(pointer_type_def::pointer_type_def)
	(reference_type_def::reference_type_def)
	(reference_type_def::get_qualified_name, strip_typedef): Use it to
	ensure that empty pointed-to-type is considered as a void type.

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