Allowed Enum variants to be individually marked as untagged (#2403)

test_suite/tests/ui/enum-representation/partially_tagged_wrong_order.rs

@@ -0,0 +1,10 @@
+use serde_derive::Serialize;
+
+#[derive(Serialize)]
+enum E {
+    #[serde(untagged)]
+    A(u8),
+    B(String),
+}
+
+fn main() {}

test_suite/tests/ui/enum-representation/partially_tagged_wrong_order.stderr

@@ -0,0 +1,5 @@
+error: all variants with the #[serde(untagged)] attribute must be placed at the end of the enum
+ --> tests/ui/enum-representation/partially_tagged_wrong_order.rs:7:5
+  |
+7 |     B(String),
+  |     ^