Add an ErrorGuaranteed to ast::TyKind::Err.

This makes it more like `hir::TyKind::Err`, and avoids a
`span_delayed_bug` call in `LoweringContext::lower_ty_direct`.

It also requires adding `ast::TyKind::Dummy`, now that
`ast::TyKind::Err` can't be used for that purpose in the absence of an
error emission.

There are a couple of cases that aren't as neat as I would have liked,
marked with `FIXME` comments.

compiler/rustc_passes/src/hir_stats.rs

@@ -616,8 +616,9 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
                 Infer,
                 ImplicitSelf,
                 MacCall,
-                Err,
-                CVarArgs
+                CVarArgs,
+                Dummy,
+                Err
             ]
         );