Add simple async drop glue generation

Explainer: https://zetanumbers.github.io/book/async-drop-design.html

https://github.com/rust-lang/rust/pull/121801

compiler/rustc_monomorphize/src/collector.rs

@@ -973,6 +973,7 @@ fn visit_instance_use<'tcx>(
             }
         }
         ty::InstanceDef::DropGlue(_, Some(_))
+        | ty::InstanceDef::AsyncDropGlueCtorShim(..)
         | ty::InstanceDef::VTableShim(..)
         | ty::InstanceDef::ReifyShim(..)
         | ty::InstanceDef::ClosureOnceShim { .. }

compiler/rustc_monomorphize/src/partitioning.rs

@@ -625,7 +625,8 @@ fn characteristic_def_id_of_mono_item<'tcx>(
                 | ty::InstanceDef::Virtual(..)
                 | ty::InstanceDef::CloneShim(..)
                 | ty::InstanceDef::ThreadLocalShim(..)
-                | ty::InstanceDef::FnPtrAddrShim(..) => return None,
+                | ty::InstanceDef::FnPtrAddrShim(..)
+                | ty::InstanceDef::AsyncDropGlueCtorShim(..) => return None,
             };
 
             // If this is a method, we want to put it into the same module as
@@ -769,7 +770,9 @@ fn mono_item_visibility<'tcx>(
     };
 
     let def_id = match instance.def {
-        InstanceDef::Item(def_id) | InstanceDef::DropGlue(def_id, Some(_)) => def_id,
+        InstanceDef::Item(def_id)
+        | InstanceDef::DropGlue(def_id, Some(_))
+        | InstanceDef::AsyncDropGlueCtorShim(def_id, _) => def_id,
 
         // We match the visibility of statics here
         InstanceDef::ThreadLocalShim(def_id) => {