Update ty::VariantDef to use IndexVec<FieldIdx, FieldDef>

And while doing the updates for that, also uses `FieldIdx` in `ProjectionKind::Field` and `TypeckResults::field_indices`.

There's more places that could use it (like `rustc_const_eval` and `LayoutS`), but I tried to keep this PR from exploding to *even more* places.

Part 2/? of https://github.com/rust-lang/compiler-team/issues/606

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1078,6 +1078,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let tail_field = def
                     .non_enum_variant()
                     .fields
+                    .raw
                     .last()
                     .expect("expected unsized ADT to have a tail field");
                 let tail_field_ty = tcx.type_of(tail_field.did);