Auto merge of #86194 - RalfJung:const-ub-hard-error, r=oli-obk

make UB during CTFE a hard error

This is a next step for https://github.com/rust-lang/rust/issues/71800. `const_err` has been a future-incompatibility lint for 4 months now since https://github.com/rust-lang/rust/pull/80394 (and err-by-default for many years before that), so I think we could try making it a proper hard error at least in some situations.

I didn't yet adjust the tests, since I first want to gauge the fall-out via crater.
Cc `@rust-lang/wg-const-eval`

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

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