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

src/value_and_place.rs

@@ -701,7 +701,8 @@ pub(crate) fn place_field(
                     };
                 }
                 ty::Adt(adt_def, substs) if layout.ty.is_simd() => {
-                    let f0_ty = adt_def.non_enum_variant().fields[0].ty(fx.tcx, substs);
+                    let f0 = &adt_def.non_enum_variant().fields[FieldIdx::from_u32(0)];
+                    let f0_ty = f0.ty(fx.tcx, substs);
 
                     match f0_ty.kind() {
                         ty::Array(_, _) => {