Auto merge of #122500 - petrochenkov:deleg, r=fmease

delegation: Support renaming, and async, const, extern "ABI" and C-variadic functions

Also allow delegating to functions with opaque types (`impl Trait`).
The delegation item will refer to the original opaque type from the callee, fresh opaque type won't be created, which seems like a reasonable behavior.
(Such delegation items will cause query cycles when used in trait impls, but it can be fixed later.)

Part of rust-lang/rust#118212.

Author: bors