Add an optional Span to BrAnon and use it to print better error for HRTB error from generator interior

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1975,6 +1975,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
     /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
     /// ```
+    #[instrument(level = "debug", skip(self), ret)]
     fn constituent_types_for_ty(
         &self,
         t: ty::Binder<'tcx, Ty<'tcx>>,