Auto merge of #103227 - lcnr:bye-bye-unevaluated-const, r=oli-obk

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`

compiler/rustc_middle/src/ty/mod.rs

@@ -683,7 +683,7 @@ pub enum PredicateKind<'tcx> {
     Coerce(CoercePredicate<'tcx>),
 
     /// Constant initializer must evaluate successfully.
-    ConstEvaluatable(ty::UnevaluatedConst<'tcx>),
+    ConstEvaluatable(ty::Const<'tcx>),
 
     /// Constants must be equal. The first component is the const that is expected.
     ConstEquate(Const<'tcx>, Const<'tcx>),