Refractor a few more types to `rustc_type_ir`
In the continuation of #79169, ~~blocked on that PR~~.
This PR:
- moves `IntVarValue`, `FloatVarValue`, `InferTy` (and friends) and `Variance`
- creates the `IntTy`, `UintTy` and `FloatTy` enums in `rustc_type_ir`, based on their `ast` and `chalk_ir` equilavents, and uses them for types in the rest of the compiler.
~~I will split up that commit to make this easier to review and to have a better commit history.~~
EDIT: done, I split the PR in commits of 200-ish lines each
r? `````@nikomatsakis````` cc `````@jackh726`````
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