Repair/improve const eval validation errors #97250

In #95377 I'm proposing that we improve UB detection in const eval by selectively enforcing validity by attempting to detect if unsafe code has been loaded. As a result of this change, programs which would have produced an invalid const item are now detected earlier, and produce a different diagnostic. This new diagnostic doesn't include the memory dump. We should probably add it or something like it back. cc @oli-obk