don't allow ZST in ScalarInt

There are several indications that we should not ZST as a ScalarInt:
- We had two ways to have ZST valtrees, either an empty `Branch` or a `Leaf` with a ZST in it.
  `ValTree::zst()` used the former, but the latter could possibly arise as well.
- Likewise, the interpreter had `Immediate::Uninit` and `Immediate::Scalar(Scalar::ZST)`.
- LLVM codegen already had to special-case ZST ScalarInt.

So instead add new ZST variants to those types that did not have other variants
which could be used for this purpose.

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -451,6 +451,10 @@ impl<'a, 'tcx> AbstractConstBuilder<'a, 'tcx> {
                 let val = ty::ValTree::from_scalar_int(lit);
                 self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
             }
+            &ExprKind::ZstLiteral { user_ty: _ } => {
+                let val = ty::ValTree::zst();
+                self.nodes.push(Node::Leaf(ty::Const::from_value(self.tcx, val, node.ty)))
+            }
             &ExprKind::NamedConst { def_id, substs, user_ty: _ } => {
                 let uneval = ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs);