Point out serde(untagged) variants which are out of order

Previously if someone wrote an enum containing:

- `A` (untagged)
- `B` (tagged)
- `C` (tagged)
- `D` (untagged)
- `E` (tagged)
- `F` (untagged)

serde_derive would produce errors referring to B and E only, saying
you're supposed to put untagged variants at the end. The choice of B and
E for this error doesn't make a lot of sense because in order to resolve
the issue, the user must either:

- move A and D down

or:

- move B, C, and E up.

This commit changes the error to appear on A and D instead.

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

@@ -1,5 +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
+ --> tests/ui/enum-representation/partially_tagged_wrong_order.rs:6:5
   |
-7 |     B(String),
+6 |     A(u8),
   |     ^