Rollup of 7 pull requests

Successful merges:

 - rust-lang#71269 (Define UB in float-to-int casts to saturate)
 - rust-lang#71591 (use new interface to create threads on HermitCore)
 - rust-lang#71819 (x.py: Give a more helpful error message if curl isn't installed)
 - rust-lang#71893 (Use the `impls` module to import pre-existing dataflow analyses)
 - rust-lang#71929 (Use -fvisibility=hidden for libunwind)
 - rust-lang#71937 (Ignore SGX on a few ui tests)
 - rust-lang#71944 (Add comment for `Ord` implementation for array)

Failed merges:

r? @ghost

…e-analysis, r=tmandry

Clean up logic around live locals in generator analysis

Resolves rust-lang#69902. Requires rust-lang#71893.

I've found it difficult to make changes in the logic around live locals in `generator/transform.rs`. It uses a custom dataflow analysis, `MaybeRequiresStorage`, that AFAICT computes whether a local is either initialized or borrowed. That analysis is using `before` effects, which we should try to avoid if possible because they are harder to reason about than ones only using the unprefixed effects. @pnkfelix has suggested removing "before" effects entirely to simplify the dataflow framework, which I might pursue someday.

This PR replaces `MaybeRequiresStorage` with a combination of the existing `MaybeBorrowedLocals` and a new `MaybeInitializedLocals`. `MaybeInitializedLocals` is just `MaybeInitializedPlaces` with a coarser resolution: it works on whole locals instead of move paths. As a result, I was able to simplify the logic in `compute_storage_conflicts` and `locals_live_across_suspend_points`.

This is not exactly equivalent to the old logic; some generators are now smaller than before. I believe this was because the old logic was too conservative, but I'm not as familiar with the constraints as the original implementers were, so I could be wrong. For example, I don't see a reason the size of the `mixed_sizes` future couldn't be 5K. It went from 7K to 6K in this PR.

r? @jonas-schievink @tmandry