bug#15666: 24.3.50; regression: `defadvice' of a special form

[Drew Adams]

> In Emacs 24 you get this:
>  (error "Advice impossible: defconst is a special form")

And the code that raises that error has this comment:

(cond
   ((special-form-p def)
    ;; Not worth the trouble trying to handle this, I think.
    (error "Advice impossible: %S is a special form" symbol))

"I think"?  That does not justify this regression.

It was worth the trouble before it was broken, i.e., for a
couple of decades.  Why go to the "trouble" of breaking it?