Auto merge of #118461 - celinval:smir-switch-targets, r=ouz-a

Change `SwitchTarget` representation in StableMIR

The new structure encodes its invariant, which reduces the likelihood of having an inconsistent representation. It is also more intuitive and user friendly.

I encapsulated the structure for now in case we decide to change it back.

### Notes:

1. I had to change the `Successors` type, since there's a conflict on the iterator type. We could potentially implement an iterator here, but I would prefer keeping it simple for now, and add a `successors_iter()` method if needed.
2. I removed `CoroutineDrop` for now since it we never create it. We can add it when we add support to other MIR stages.

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