Update to rustc sanity check branch

[oli-obk]

to be merged after rust-lang/rust#51361 hits nightly

[RalfJung]

I updated this to also fix the breakage from promoteds being a place.

However, two compile-fail tests fail: A bad bool is no longer detected on an if, and modifying_constants.rs also just runs to completion.

[RalfJung]

Yeah the bool part is ensure_valid_value being removed.

I have plans for a more consistent version of this so I guess it's okay for that to be removed, but why is it not even detected when an if actually happens?

EDIT: Ah, the MIR actually uses switchInt(move _2) -> [false: bb2, otherwise: bb1] so it does not care.

[RalfJung]

Note to self and everybody whom it concerns: Do NOT force-push to this branch or delete it before it was merged! It is referenced by rustc.

(Might be worth stating that also in the future as it's kind of not clear just looking at this PR.^^)