Use the correct field to copy/dispose a __block variable.

We were previously hard-coding a particular field index.  This was
fine before (because we were obviously guaranteed the presence
of a copy/dispose member) except for (1) alignment padding and
(2) future extensions adding extra members to the header, such
as the extended-layout pointer.

Note that we only introduce the extended-layout pointer in the
presence of structs.  (We also seem to be introducing it even
in the presence of an all-non-object layout, but that's a
different potential issue.)