bug#67005: 30.0.50; improve nadivce/comp/trampoline handling

[Stefan Monnier]

>> +  (when purify-flag
>> +    (error "Invalid pre-dump advice on %s" symbol))
>>    (let* ((f (symbol-function symbol))
>>       (nf (advice--normalize symbol f)))
>>      (unless (eq f nf) (fset symbol nf))
>
> The patch LGTM thanks, just one question: should we guard in advice.el
> as well just to stay on the safe side?

Since `defadvice` is now marked as obsolete, it seems highly unlikely
that we'd end up adding an old-style advice to a preloaded file.

But this reminds me that some people do their own preload/dump,
sometimes with a crapload of extra packages, in which case the
probability that some of them use advice-add is rather high.
IIUC the the above `error` is not technically indispensable (the code
will still work mostly right, beside some issues about the docstrings
that affect only actual C-code subrs and not native-compiled subrs), so
it would be better to demote the above `error` to a warning.


        Stefan