ir: Add missing virtual methods overloads

ir: Add missing virtual methods overloads

Fedora Rawhide recently moved to GCC 13 and so its emitting new
warnings about libabigail's code base, and rightly so.

This patch thus adds some missing virtual method overloads that are
spotted by GCC 13.

	* include/abg-ir.h (type_decl::operator!=): Declare missing
	virtual overloads.
	(array_type_def::subrange_type::operator!=): Likewise.
	(template_decl::operator==): Likewise.
	(type_tparameter::operator==): Likewise.
	(class_decl::operator==): Likewise.
	(union_decl::operator==): Likewise.
	(member_class_template::operator==): Likewise.
	* src/abg-ir.cc (type_decl::operator!=)
	(array_type_def::subrange_type::operator!=)
	(class_decl::operator==, member_class_template::operator==)
	(union_decl::operator==, template_decl::operator==)
	(type_tparameter::operator==, type_tparameter::operator==)
	(template_tparameter::operator==): Define new virtual overloads.

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