Auto merge of #115748 - RalfJung:post-mono, r=oli-obk

move required_consts check to general post-mono-check function

This factors some code that is common between the interpreter and the codegen backends into shared helper functions. Also as a side-effect the interpreter now uses the same `eval` functions as everyone else to get the evaluated MIR constants.

Also this is in preparation for another post-mono check that will be needed for (the current hackfix for) https://github.com/rust-lang/rust/issues/115709: ensuring that all locals are dynamically sized.

I didn't expect this to change diagnostics, but it's just cycle errors that change.

r? `@oli-obk`

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -956,8 +956,10 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         use rustc_middle::mir::interpret::ErrorHandled;
         match self.infcx.try_const_eval_resolve(param_env, unevaluated, ty, None) {
             Ok(ct) => Some(ct),
-            Err(ErrorHandled::Reported(e)) => Some(ty::Const::new_error(self.tcx(), e.into(), ty)),
-            Err(ErrorHandled::TooGeneric) => None,
+            Err(ErrorHandled::Reported(e, _)) => {
+                Some(ty::Const::new_error(self.tcx(), e.into(), ty))
+            }
+            Err(ErrorHandled::TooGeneric(_)) => None,
         }
     }