c++: explicit object cleanups
The FIXME in xobj_iobj_parameters_correspond was due to expecting
TYPE_MAIN_VARIANT to be the same for all equivalent types, which is not the
case. And I adjusted some comments that I disagree with; the iobj parameter
adjustment only applies to overload resolution, we can handle that in
cand_parms_match (and I have WIP for that).
gcc/cp/ChangeLog:
* call.cc (build_over_call): Refactor handle_arg lambda.
* class.cc (xobj_iobj_parameters_correspond): Fix FIXME.
* method.cc (defaulted_late_check): Adjust comments.