* ipa-devirt.c (type_pair, default_hashset_traits): New types.
(odr_types_equivalent_p): Use pair hash.
(odr_subtypes_equivalent_p): Likewise, do structural compare
on ODR types that may be mismatched.
(warn_odr): Support warning when only one field is given.
(odr_types_equivalent_p): Strenghten comparsions made;
support VOIDtype.
(add_type_duplicate): Update VISITED hash set.