Safe Transmute: Refactor error handling and Answer type

- Create `Answer` type that is not just a type alias of `Result`
- Remove a usage of `map_layouts` to make the code easier to read
- Don't hide errors related to Unknown Layout when computing transmutability

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -668,6 +668,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         scope: Ty<'tcx>,
         assume: rustc_transmute::Assume,
     ) -> Result<Certainty, NoSolution> {
+        use rustc_transmute::Answer;
         // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             ObligationCause::dummy(),
@@ -675,11 +676,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             scope,
             assume,
         ) {
-            Ok(None) => Ok(Certainty::Yes),
-            Err(_)
-            | Ok(Some(rustc_transmute::Condition::IfTransmutable { .. }))
-            | Ok(Some(rustc_transmute::Condition::IfAll(_)))
-            | Ok(Some(rustc_transmute::Condition::IfAny(_))) => Err(NoSolution),
+            Answer::Yes => Ok(Certainty::Yes),
+            Answer::No(_) | Answer::If(_) => Err(NoSolution),
         }
     }
 }