derive: Emit only PartialOrd::partial_cmp for simple enums

Using the same logic as for `PartialEq`, when possible define only
`partial_cmp` and leave `lt, le, gt, ge` to their default
implementations. This works well for c-like enums.

src/libsyntax_ext/deriving/generic/mod.rs

@@ -1638,3 +1638,21 @@ pub fn cs_same_method<F>(f: F,
         }
     }
 }
+
+/// Return true if the type has no value fields
+/// (for an enum, no variant has any fields)
+pub fn is_type_without_fields(item: &Annotatable) -> bool {
+    if let Annotatable::Item(ref item) = *item {
+        match item.node {
+            ast::ItemKind::Enum(ref enum_def, _) => {
+                enum_def.variants.iter().all(|v| v.node.data.fields().is_empty())
+            }
+            ast::ItemKind::Struct(ref variant_data, _) => {
+                variant_data.fields().is_empty()
+            }
+            _ => false
+        }
+    } else {
+        false
+    }
+}