fn build_union_fields_for_niche_tag_enum<'ll, 'tcx>(
    cx: &CodegenCx<'ll, 'tcx>,
    enum_adt_def: AdtDef<'tcx>,
    enum_type_and_layout: TyAndLayout<'tcx>,
    enum_type_di_node: &'ll DIType,
    dataful_variant_index: VariantIdx,
    variant_indices: &mut dyn Iterator<Item = VariantIdx>,
    tag_field: usize
) -> SmallVec<[&'ll DIType; 16]>