Port #[ffi_pure] to the new attribute system

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -254,6 +254,9 @@ pub enum AttributeKind {
     /// Represents `#[ffi_const]`.
     FfiConst(Span),
 
+    /// Represents `#[ffi_pure]`.
+    FfiPure(Span),
+
     /// Represents `#[ignore]`
     Ignore {
         span: Span,

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -28,6 +28,7 @@ impl AttributeKind {
             ExportName { .. } => Yes,
             ExportStable => No,
             FfiConst(..) => No,
+            FfiPure(..) => No,
             Ignore { .. } => No,
             Inline(..) => No,
             LinkName { .. } => Yes,