fn build_union_fields_for_enum<'ll, 'tcx>(
    cx: &CodegenCx<'ll, 'tcx>,
    enum_adt_def: AdtDef<'tcx>,
    enum_type_and_layout: TyAndLayout<'tcx>,
    enum_type_di_node: &'ll DIType,
    variant_indices: impl Iterator<Item = VariantIdx> + Clone,
    tag_field: usize,
    untagged_variant_index: Option<VariantIdx>
) -> SmallVec<[&'ll DIType; 16]>