Test that Self works in filters

Author: mejrs

tests/ui/on-unimplemented/use_self_no_underscore.rs

@@ -0,0 +1,6 @@
+//@ check-pass
+#![feature(rustc_attrs)]
+#![crate_type = "lib"]
+// Support `Self`, not just `_Self`
+#[rustc_on_unimplemented(on(Self = "u8"), message = "message")]
+pub trait Foo {}