Rollup merge of #118516 - celinval:smir-variants, r=ouz-a

Add ADT variant infomation to StableMIR and finish implementing TyKind::internal()

Introduce a `VariantDef` type and a mechanism to retrieve the definition from an `AdtDef`.

The `VariantDef` representation itself is just a combination of `AdtDef` and `VariantIdx`, which allow us to retrieve further information of a variant. I don't think we need to cache extra information for now, and we can translate on an on demand manner. I am  leaving the fields public today due to https://github.com/rust-lang/project-stable-mir/issues/56, but they shouldn't. For this PR, I've only added a method to retrieve the variant name, and its fields. I also added an implementation of `RustcInternal` that allow users to retrieve more information using Rust internal APIs.

I have also finished the implementation of `RustcInternal` for `TyKind` which fixes https://github.com/rust-lang/project-stable-mir/issues/46.

## Motivation

Both of these changes are needed in order to properly interpret things like projections. For example,
- The variant definition is used to find out which variant we are downcasting to.
- Being able to create `Ty` from `TyKind` helps for example processing each stage of a projection, like the code in `place.ty()`.

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