Port #[may_dangle] to the new attribute system

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -34,6 +34,7 @@ pub(crate) mod deprecation;
 pub(crate) mod inline;
 pub(crate) mod lint_helpers;
 pub(crate) mod repr;
+pub(crate) mod semantics;
 pub(crate) mod stability;
 pub(crate) mod transparency;
 pub(crate) mod util;