Rename empty/bang to never

Split Ty::is_empty method into is_never and is_uninhabited

src/libsyntax/fold.rs

@@ -373,7 +373,7 @@ pub fn noop_fold_ty<T: Folder>(t: P<Ty>, fld: &mut T) -> P<Ty> {
                     decl: fld.fold_fn_decl(decl)
                 }))
             }
-            TyKind::Empty => node,
+            TyKind::Never => node,
             TyKind::Tup(tys) => TyKind::Tup(tys.move_map(|ty| fld.fold_ty(ty))),
             TyKind::Paren(ty) => TyKind::Paren(fld.fold_ty(ty)),
             TyKind::Path(qself, path) => {