Add a way to add constructors for `rustc_type_ir` types
Introduces a module called `rustc_type_ir`, in which we can place traits which are named `Ty`/`Region`/`Const`/etc. which expose constructors for the `rustc_type_ir` types. This means we can construct things `Interner::Ty` with `Ty::new_x(...)`, which is needed to uplift the new trait solver into an interner-agnostic crate.
These traits are placed into a *separate* module because they're only intended to be used in interner-agnostic code, and they should mirror the constructors that are provided by the inherent constructor methods in `rustc_middle`.
Putting this up for vibe-check mostly. I haven't copied over any of the type constructors, except for one to create bound types for use in the canonicalizer.
r? lcnr
Matthias Krüger
e7bb224219