Add missing deep equality operator for pointer and reference types

Add missing deep equality operator for pointer and reference types

I noticed that abigail::ir::pointer_type_def_sptr and
abigail::ir::reference_type_def_sptr did not have any free form
operator '==' defined.  So writing a == b with a and b being either
pointer_type_def_sptr or reference_type_def_sptr was using pointer
value comparison, as opposed to deeply comparing the pointer and
reference instances.

This patch adds those two missing operators.

	* include/abg-ir.h (pointer_type_def::operator==): Add an overload
	for pointer_type_def.
	(reference_type_def::operator==) Add an overload for
	reference_type_def.
	(operator==): Add an overload for pointer_type_def_sptr and
	reference_type_def_sptr.
	* src/abg-ir.cc (pointer_type_def::operator==): Make the overload
	for type_base& use the overload for decl_base&.  Add a new
	overload for pointer_type_def& and make is use the overload for
	decl_base& too.
	(operator==): Add free form overloads for pointer_type_def& and
	reference_type_def&.
	(reference_type_def::operator==): Add comments. Add an overload
	for reference_type_def&.

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