Fix async drop multi crate crash

[ValorZard]

fixes #142572
Again, I've been talking in the Zulip about this: #wg-async > is there a way I can help with Async Drop? @ 💬

TL;DR: This is going to be a bit of a refactor, hold your horses

Also, I might end up splitting this into two PRs, so I can first merge my elaborate_drop.rs refactor, and then my fix for the crash later, but IDK yet

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @davidtwco

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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[ValorZard]

r? @oli-obk

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[oli-obk]

could do if self.tcx().features().staged_api() { span_bug!(some_span, "don't use async drop in libstd, it becomes insta-stable") } (and similarly at the other use of async_drop in adt_constructor

[oli-obk]

if you write fixes #142572 in the main PR message, the issue will automatically be closed when the PR merges