make UB during CTFE a hard error

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -525,6 +525,7 @@ pub fn is_hard_err(&self) -> bool {
         use InterpError::*;
         match *self {
             MachineStop(ref err) => err.is_hard_err(),
+            InterpError::UndefinedBehavior(_) => true,
             _ => false,
         }
     }