def_collector: Do not forget to save indices of fields with multiple attributes

src/test/ui/attributes/unnamed-field-attributes-dup.rs

@@ -0,0 +1,11 @@
+// Duplicate non-builtin attributes can be used on unnamed fields.
+
+// check-pass
+
+struct S (
+    #[rustfmt::skip]
+    #[rustfmt::skip]
+    u8
+);
+
+fn main() {}