Port #[rustc_unsafe_specialization_marker] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -365,6 +365,9 @@ pub enum AttributeKind {
     /// Represents `#[type_const]`.
     TypeConst(Span),
 
+    /// Represents `#[rustc_unsafe_specialization_marker]`.
+    UnsafeSpecializationMarker(Span),
+
     /// Represents `#[used]`
     Used { used_by: UsedBy, span: Span },
     // tidy-alphabetical-end

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -61,6 +61,7 @@ impl AttributeKind {
             TargetFeature(..) => No,
             TrackCaller(..) => Yes,
             TypeConst(..) => Yes,
+            UnsafeSpecializationMarker(..) => No,
             Used { .. } => No,
             // tidy-alphabetical-end
         }