Do not attemt to continue parsing after pub ident

Try to identify the following code in order to provide better
diagnostics, but return the error to bail out early during the parse.

src/librustc_resolve/lib.rs

@@ -1947,8 +1947,7 @@ impl<'a> Resolver<'a> {
                 }
             }
 
-            ItemKind::ExternCrate(_) | ItemKind::MacroDef(..) | ItemKind::GlobalAsm(_) |
-            ItemKind::Placeholder => {
+            ItemKind::ExternCrate(_) | ItemKind::MacroDef(..) | ItemKind::GlobalAsm(_) => {
                 // do nothing, these are just around to be encoded
             }