Port #[marker] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -294,6 +294,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_macro_transparency]`.
     MacroTransparency(Transparency),
 
+    /// Represents `#[marker]`.
+    Marker(Span),
+
     /// Represents [`#[may_dangle]`](https://std-dev-guide.rust-lang.org/tricky/may-dangle.html).
     MayDangle(Span),
 

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -40,6 +40,7 @@ impl AttributeKind {
             LinkSection { .. } => No,
             LoopMatch(..) => No,
             MacroTransparency(..) => Yes,
+            Marker(..) => No,
             MayDangle(..) => No,
             MustUse { .. } => Yes,
             Naked(..) => No,