Auto merge of #79635 - lcnr:const-eval-idk, r=oli-obk

Auto merge of #79635 - lcnr:const-eval-idk, r=oli-obk

const_evaluatable_checked: fix occurs check

fixes #79615

this is kind of a hack because we use `TypeRelation` for both the `Generalizer` and the `ConstInferUnifier` but i am not sure if there is a useful way to disentangle this without unnecessarily duplicating some code.

The error in the added test is kind of unavoidable until we erase the unused substs of `ConstKind::Unevaluated`. We talked a bit about this in the cg lazy norm meeting (https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/lazy_normalization_consts)