Auto merge of #149948 - WaffleLapkin:dereferenceablen't, r=RalfJung

Stop applying `dereferenceable(n)` to return types

It looks like the semantics of `dereferenceable(n)` on return types is "dereferenceable until the end of the program", which is not sound for how we were using it. See [dereferenceable on return type](https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/LLVM.20dereferenceable.20on.20return.20type/with/563001493) zulip thread.

cc `@rust-lang/opsem` `@nikic`

Author: bors