Expand async drops during drop elaboration

[cjgillot]

Async drops implemented by #123948 are expanded in await loops inside StateTransform. I find this odd, as StateTransform is tasked with removing yields from MIR.

This PR moves this async drop -> loop transformation to drop elaboration.

The first few commits create many mir-opt tests to gauge the effect of following commits. I can easily trim those.
The next few commits are from #156422.
Then come a few commits performing elementary simplifications to drop elaboration code.

The most important commit is Expand async drops during drop elaboration. It performs the main change and attempts to document what I had to understand doing that.

The last commit is a cleanup.

This is a large PR, but most of the diff is in generated files. I can easily split them, each commit compiles and passes tests on its own.

cc @zetanumbers

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

The Cranelift subtree was changed

cc @bjorn3

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

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[bjorn3]

Is there some relation to #156495 or would that PR be orthogonal to this one?

[cjgillot]

Is there some relation to #156495 or would that PR be orthogonal to this one?

It should be orthogonal.

[zetanumbers]

uuh
cc @azhogin