Change tag_field to FieldIdx in Variants::Multiple

It was already available as a generic parameter anyway, and it's not like we'll ever put a tag in the 5-billionth field.

compiler/rustc_transmute/src/layout/tree.rs

@@ -451,7 +451,7 @@ pub(crate) mod rustc {
 
                     // For enums (but not coroutines), the tag field is
                     // currently always the first field of the layout.
-                    assert_eq!(*tag_field, 0);
+                    assert_eq!(*tag_field, FieldIdx::ZERO);
 
                     let variants = def.discriminants(cx.tcx()).try_fold(
                         Self::uninhabited(),