stop using `ty::UnevaluatedConst` directly best reviewed commit by commit. simplifies #99798 because we now don't have to expand `ty::UnevaluatedConst` to `ty::Const`. I also remember some other places where using `ty::UnevaluatedConst` directly was annoying and caused issues, though I don't quite remember what they were rn '^^ r? `@oli-obk` cc `@JulianKnodt`