Rewrite variant, also PR libstdc++/85517

* include/std/variant (__do_visit): New.
(__variant_cast): Likewise.
(__variant_cookie): Likewise.
(__erased_*): Remove.
(_Variant_storage::_S_vtable): Likewise.
(_Variant_storage::__M_reset_impl): Adjust to use __do_visit.
(_Variant_storage::__M_reset): Adjust.
(__variant_construct): New.
(_Copy_ctor_base(const _Copy_ctor_base&)): Adjust to use
__variant_construct.
(_Move_ctor_base(_Move_ctor_base&&)): Likewise.
(_Move_ctor_base::__M_destructive_copy): New.
(_Move_ctor_base::__M_destructive_move): Adjust to use
__variant_construct.
(_Copy_assign_base::operator=): Adjust to use __do_visit.
(_Copy_assign_alias): Adjust to check both copy assignment
and copy construction for triviality.
(_Move_assign_base::operator=): Adjust to use __do_visit.
(_Multi_array): Add support for visitors that accept and return
a __variant_cookie.
(__gen_vtable_impl::_S_apply_all_alts): Likewise.
(__gen_vtable_impl::_S_apply_single_alt): Likewise.
(__gen_vtable_impl::__element_by_index_or_cookie): New. Generate
a __variant_cookie temporary for a variant that is valueless and..
(__gen_vtable_impl::__visit_invoke): ..adjust here.
(__gen_vtable::_Array_type): Conditionally make space for
the __variant_cookie visitor case.
(__variant_construct_by_index): New.
(get_if): Adjust to use std::addressof.
(relops): Adjust to use __do_visit.
(variant): Add __variant_cast and __variant_construct_by_index
as friends.
(variant::emplace): Use _M_reset() and __variant_construct_by_index
instead of self-destruction.
(variant::swap): Adjust to use __do_visit.
(visit): Reimplement in terms of __do_visit.
(__variant_hash_call_base_impl::operator()): Adjust to use __do_visit.
* testsuite/20_util/variant/compile.cc: Adjust.
* testsuite/20_util/variant/run.cc: Likewise.

From-SVN: r269422