Auto merge of #83207 - oli-obk:valtree2, r=lcnr

normalize mir::Constant differently from ty::Const in preparation for valtrees

Valtrees are unable to represent many kind of constant values (this is on purpose). For constants that are used at runtime, we do not need a valtree representation and can thus use a different form of evaluation. In order to make this explicit and less fragile, I added a `fold_constant` method to `TypeFolder` and implemented it for normalization. Normalization can now, when it wants to eagerly evaluate a constant, normalize `mir::Constant` directly into a `mir::ConstantKind::Val` instead of relying on the `ty::Const` evaluation.

In the future we can get rid of the `ty::Const` in there entirely and add our own `Unevaluated` variant to `mir::ConstantKind`. This would allow us to remove the `promoted` field from `ty::ConstKind::Unevaluated`, as promoteds can never occur in the type system.

cc `@rust-lang/wg-const-eval`

r? `@lcnr`

compiler/rustc_middle/src/query/mod.rs

@@ -1486,6 +1486,13 @@ rustc_queries! {
         desc { "normalizing `{}`", goal.value }
     }
 
+    /// Do not call this query directly: invoke `normalize_erasing_regions` instead.
+    query normalize_mir_const_after_erasing_regions(
+        goal: ParamEnvAnd<'tcx, mir::ConstantKind<'tcx>>
+    ) -> mir::ConstantKind<'tcx> {
+        desc { "normalizing `{}`", goal.value }
+    }
+
     query implied_outlives_bounds(
         goal: CanonicalTyGoal<'tcx>
     ) -> Result<