Port #[rustc_as_ptr] to the new attribute system

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -29,6 +29,7 @@ pub(crate) mod allow_unstable;
 pub(crate) mod cfg;
 pub(crate) mod confusables;
 pub(crate) mod deprecation;
+pub(crate) mod lint_helpers;
 pub(crate) mod repr;
 pub(crate) mod stability;
 pub(crate) mod transparency;