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_ty_utils/src/layout.rs

@@ -932,7 +932,7 @@ fn variant_info_for_coroutine<'tcx>(
             // However, if the discriminant is placed past the end of the variant, then we need
             // to factor in the size of the discriminant manually. This really should be refactored
             // better, but this "works" for now.
-            if layout.fields.offset(tag_field) >= variant_size {
+            if layout.fields.offset(tag_field.as_usize()) >= variant_size {
                 variant_size += match tag_encoding {
                     TagEncoding::Direct => tag.size(cx),
                     _ => Size::ZERO,