Auto merge of #134625 - compiler-errors:unsafe-binders-ty, r=oli-obk

Begin to implement type system layer of unsafe binders

Mostly TODOs, but there's a lot of match arms that are basically just noops so I wanted to split these out before I put up the MIR lowering/projection part of this logic.

r? oli-obk

Tracking:

- rust-lang/rust#130516