Auto merge of #124662 - zetanumbers:needs_async_drop, r=oli-obk

Implement `needs_async_drop` in rustc and optimize async drop glue

This PR expands on #121801 and implements `Ty::needs_async_drop` which works almost exactly the same as `Ty::needs_drop`, which is needed for #123948.

Also made compiler's async drop code to look more like compiler's regular drop code, which enabled me to write an optimization where types which do not use `AsyncDrop` can simply forward async drop glue to `drop_in_place`. This made size of the async block from the [async_drop test](https://github.com/zetanumbers/rust/blob/67980dd6fb11917d23d01a19c2cf4cfc3978aac8/tests/ui/async-await/async-drop.rs) to decrease by 12%.

Author: bors

tests/pass/async-drop.rs

@@ -1,6 +1,11 @@
 //@revisions: stack tree
 //@compile-flags: -Zmiri-strict-provenance
 //@[tree]compile-flags: -Zmiri-tree-borrows
+
+// WARNING: If you would ever want to modify this test,
+// please consider modifying rustc's async drop test at
+// `tests/ui/async-await/async-drop.rs`.
+
 #![feature(async_drop, impl_trait_in_assoc_type, noop_waker, async_closure)]
 #![allow(incomplete_features, dead_code)]