Port #[rustc_specialization_trait] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -343,6 +343,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_skip_during_method_dispatch]`.
     SkipDuringMethodDispatch { array: bool, boxed_slice: bool, span: Span },
 
+    /// Represents `#[rustc_specialization_trait]`.
+    SpecializationTrait(Span),
+
     /// Represents `#[stable]`, `#[unstable]` and `#[rustc_allowed_through_unstable_modules]`.
     Stability {
         stability: Stability,

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -55,6 +55,7 @@ impl AttributeKind {
             RustcLayoutScalarValidRangeStart(..) => Yes,
             RustcObjectLifetimeDefault => No,
             SkipDuringMethodDispatch { .. } => No,
+            SpecializationTrait(..) => No,
             Stability { .. } => Yes,
             StdInternalSymbol(..) => No,
             TargetFeature(..) => No,