Don't ICE on anonymous struct in enum variant

tests/ui/union/unnamed-fields/anon-struct-in-enum-issue-121446.rs

@@ -0,0 +1,11 @@
+#![crate_type = "lib"]
+#![feature(unnamed_fields)]
+#![allow(unused, incomplete_features)]
+
+enum K {
+    M {
+        _ : struct { field: u8 },
+        //~^ error: unnamed fields are not allowed outside of structs or unions
+        //~| error: anonymous structs are not allowed outside of unnamed struct or union fields
+    }
+}