Port #[rustc_coinductive] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -211,6 +211,9 @@ pub enum AttributeKind {
         span: Span,
     },
 
+    /// Represents `#[rustc_coinductive]`.
+    Coinductive(Span),
+
     /// Represents `#[cold]`.
     Cold(Span),
 

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -18,6 +18,7 @@ impl AttributeKind {
             AllowInternalUnstable(..) => Yes,
             AsPtr(..) => Yes,
             BodyStability { .. } => No,
+            Coinductive(..) => No,
             Cold(..) => No,
             Confusables { .. } => Yes,
             ConstContinue(..) => No,