Properly fix the personality_fn setting in MIR trans

[nagisa]

#37862 has landed despite my repeated warnings this is a hack and should be fixed properly. This issue is a reminder for self to fix this properly once the larger PRs in queue land (the old trans cleanup and i128).

The fix would likely be as simple as fixing init_cpad function and reverting the whole patch from the linked PR.

[Mark-Simulacrum]

Just leaving a note here so it's easier to find for whoever works on this: After #38302, init_cpad will not exist, but will be inlined into trans_mir.