Port #[type_const] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -359,6 +359,9 @@ pub enum AttributeKind {
     /// Represents `#[track_caller]`
     TrackCaller(Span),
 
+    /// Represents `#[type_const]`.
+    TypeConst(Span),
+
     /// Represents `#[used]`
     Used { used_by: UsedBy, span: Span },
     // tidy-alphabetical-end

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -59,6 +59,7 @@ impl AttributeKind {
             StdInternalSymbol(..) => No,
             TargetFeature(..) => No,
             TrackCaller(..) => Yes,
+            TypeConst(..) => Yes,
             Used { .. } => No,
             // tidy-alphabetical-end
         }