Not every entry in a C++ vtable corresponds to a function that we want
to import. For the holes, leave a `SemIR::InstId::None` in the vtable.
Also mark vtables that extend a C++ vtable as being non-Carbon-native so
we don't try to lower them (and crash on the `None` entries).
In particular, we leave holes for destructors, since we don't have
destructor declarations on the Carbon side that need to override them.
Richard Smith
c9d8b59bbd