Specialize ZeroSized constants

ZeroSized constants can be represented as `mir::Const::Val` even if
their layout is not yet known. In those cases, CrateItem::body() was
crashing when trying to convert a `ConstValue::ZeroSized` into its
stable counterpart `ConstantKind::Allocated`.

Instead, we now map `ConstValue::ZeroSized` into a new variant:
`ConstantKind::ZeroSized`.

compiler/stable_mir/src/ty.rs

@@ -444,6 +444,9 @@ pub enum ConstantKind {
     Allocated(Allocation),
     Unevaluated(UnevaluatedConst),
     Param(ParamConst),
+    /// Store ZST constants.
+    /// We have to special handle these constants since its type might be generic.
+    ZeroSized,
 }
 
 #[derive(Clone, Debug)]