Enable co-induction support for Safe Transmute

This patch adds the `#[rustc_coinductive]` annotation to
`BikeshedIntrinsicFrom`, so that it's possible to compute transmutability for
recursive types.

core/src/mem/transmutability.rs

@@ -9,6 +9,7 @@ use crate::marker::ConstParamTy;
 #[lang = "transmute_trait"]
 #[cfg_attr(not(bootstrap), rustc_deny_explicit_impl(implement_via_object = false))]
 #[cfg_attr(bootstrap, rustc_deny_explicit_impl)]
+#[rustc_coinductive]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,