Erase late bound regions from `Instance::fn_sig()` and add a few more details to StableMIR APIs
The Instance `fn_sig()` still included a late bound regions which needed a new compiler function in order to be erased. I've also bundled the following small fixes in this PR, let me know if you want me to isolate any of them.
- Add missing `CoroutineKind::AsyncGen`.
- Add optional spread argument to function body which is needed to properly analyze compiler shims.
- Add a utility method to iterate over all locals together with their declaration.
- Add a method to get the description of `AssertMessage`*.
* For the last one, we could consider eventually calling the internal `AssertKind::description()` to avoid code duplication. However, we still don't have ways to convert `AssertMessage`, `Operand`, `Place` and others, in order to use that. The other downside of using the internal method is that it will panic for some of the variants.
r ? `@ouz-a`
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