Port #[rustc_unsafe_specialization_marker] to the new attribute system

compiler/rustc_attr_parsing/src/context.rs

@@ -46,6 +46,7 @@ use crate::attributes::test_attrs::IgnoreParser;
 use crate::attributes::traits::{
     CoinductiveParser, ConstTraitParser, DenyExplicitImplParser, DoNotImplementViaObjectParser,
     SkipDuringMethodDispatchParser, SpecializationTraitParser, TypeConstParser,
+    UnsafeSpecializationMarkerParser,
 };
 use crate::attributes::transparency::TransparencyParser;
 use crate::attributes::{AttributeParser as _, Combine, Single, WithoutArgs};
@@ -171,6 +172,7 @@ attribute_parsers!(
         Single<WithoutArgs<StdInternalSymbolParser>>,
         Single<WithoutArgs<TrackCallerParser>>,
         Single<WithoutArgs<TypeConstParser>>,
+        Single<WithoutArgs<UnsafeSpecializationMarkerParser>>,
         // tidy-alphabetical-end
     ];
 );