Rollup merge of #133610 - camelid:move-from_anon_const, r=BoxyUwU

Move `Const::{from_anon_const,try_from_lit}` to hir_ty_lowering

Fixes #128176.
This accomplishes one of the followup items from #131081.

These operations are much more about lowering the HIR than about
`Const`s themselves. They fit better in hir_ty_lowering with
`lower_const_arg` (formerly `Const::from_const_arg`) and the rest.

To accomplish this, `const_evaluatable_predicates_of` had to be changed
to not use `from_anon_const` anymore. Instead of visiting the HIR and
lowering anon consts on the fly, it now visits the `rustc_middle::ty`
data structures instead and directly looks for `UnevaluatedConst`s. This
approach was proposed in:
https://github.com/rust-lang/rust/pull/131081#discussion_r1821189257

r? `@BoxyUwU`

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -13,6 +13,7 @@ use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_macros::LintDiagnostic;
+use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
@@ -1170,19 +1171,13 @@ fn check_type_defn<'tcx>(
 
             // Explicit `enum` discriminant values must const-evaluate successfully.
             if let ty::VariantDiscr::Explicit(discr_def_id) = variant.discr {
-                let cause = traits::ObligationCause::new(
-                    tcx.def_span(discr_def_id),
-                    wfcx.body_def_id,
-                    ObligationCauseCode::Misc,
-                );
-                wfcx.register_obligation(Obligation::new(
-                    tcx,
-                    cause,
-                    wfcx.param_env,
-                    ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(
-                        ty::Const::from_anon_const(tcx, discr_def_id.expect_local()),
-                    ))),
-                ));
+                match tcx.const_eval_poly(discr_def_id) {
+                    Ok(_) => {}
+                    Err(ErrorHandled::Reported(..)) => {}
+                    Err(ErrorHandled::TooGeneric(sp)) => {
+                        span_bug!(sp, "enum variant discr was too generic to eval")
+                    }
+                }
             }
         }