Change call_site_target to use custom type and enum

Change call_site_target to use custom type and enum

call_site_target reuses field_loc_kind and field_location.  However,
it has never used the full range of the field_loc_kind enum.  In a
subsequent patch, I plan to add a new 'kind' here, so it seemed best
to avoid this reuse and instead introduce new types here.