Port #[rustc_pub_transparent] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -240,6 +240,9 @@ pub enum AttributeKind {
     /// Represents `#[optimize(size|speed)]`
     Optimize(OptimizeAttr, Span),
 
+    /// Represents `#[rustc_pub_transparent]` (used by the `repr_transparent_external_private_fields` lint).
+    PubTransparent(Span),
+
     /// Represents [`#[repr]`](https://doc.rust-lang.org/stable/reference/type-layout.html#representations).
     Repr(ThinVec<(ReprAttr, Span)>),