Introduce sym::dummy and Ident::dummy.

The idea is to identify cases of symbols/identifiers that are not
expected to be used. There isn't a perfectly sharp line between "dummy"
and "not dummy", but I think it's useful nonetheless.

compiler/rustc_ast/src/mut_visit.rs

@@ -1948,7 +1948,7 @@ impl DummyAstNode for Item {
                 span: Default::default(),
                 tokens: Default::default(),
             },
-            ident: Ident::empty(),
+            ident: Ident::dummy(),
             kind: ItemKind::ExternCrate(None),
             tokens: Default::default(),
         }