Port #[rustc_std_internal_symbol] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -335,6 +335,9 @@ pub enum AttributeKind {
         span: Span,
     },
 
+    /// Represents `#[rustc_std_internal_symbol]`.
+    StdInternalSymbol(Span),
+
     /// Represents `#[target_feature(enable = "...")]`
     TargetFeature(ThinVec<(Symbol, Span)>, Span),
 

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -51,6 +51,7 @@ impl AttributeKind {
             RustcObjectLifetimeDefault => No,
             SkipDuringMethodDispatch { .. } => No,
             Stability { .. } => Yes,
+            StdInternalSymbol(..) => No,
             TargetFeature(..) => No,
             TrackCaller(..) => Yes,
             Used { .. } => No,