Mark __opstate classes as final instead of adding virtual destructors

[janciesko]

• Adds missing virtual modifiers to destructors in virtual classes.
• Fixes warning [1] with clang++16.

[1]

warning: destructor called on non-final 'exec::__repeat_n::__opstate<stdexec::(anonymous namespace)::__sexpr<stdexec::(lambda at /ascldap/users/jciesko/RAndD/stdexec/include/stdexec/__detail/__basic_sender.hpp:56:53){}>, nvexec::_strm::propagate_receiver_t<nvexec::_strm::stream_enqueue_receiver<stdexec::__env::env<stdexec::__env::prop<nvexec::_strm::get_stream_provider_t, nvexec::_strm::stream_provider_t *>, stdexec::__env::__fwd<stdexec::__sync_wait::__env>::__t>, nvexec::variant_t<cuda::std::tuple<nvexec::_strm::set_noop>, cuda::std::tuple<stdexec::__rcvrs::set_error_t, cudaError>, cuda::std::tuple<stdexec::__rcvrs::set_error_t, std::__exception_ptr::exception_ptr>, cuda::std::tuple<stdexec::__rcvrs::set_stopped_t>, cuda::std::tuple<stdexec::__rcvrs::set_value_t>>>>::__t>' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]
        __location->~_Tp();
        ^

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[ericniebler]

my preference would be to make the __opstate class final rather than adding a bunch of vtable entries that won't be used.

[janciesko]

Good point

[janciesko]

Updated.

[ericniebler]

/ok to test b444fb8