[Bug c++/84843] [8 Regression] C++ ICE on builtin redefinition since r258391

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84843
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
So, do we want to return newdecl unconditionally and scrap the olddecl
(builtin)
binding somehow?  For -fpermissive, of course?