add FIXME to EnumTagInfo

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -1637,6 +1637,9 @@ fn create_member_descriptions(&self, cx: &CodegenCx<'ll, 'tcx>) -> Vec<MemberDes
     }
 }
 
+// FIXME: terminology here should be aligned with `abi::TagEncoding`.
+// `OptimizedTag` is `TagEncoding::Niche`, `RegularTag` is `TagEncoding::Direct`.
+// `NoTag` should be removed; users should use `Option<EnumTagInfo>` instead.
 #[derive(Copy, Clone)]
 enum EnumTagInfo<'ll> {
     RegularTag { tag_field: Field, tag_type_metadata: &'ll DIType },