[Bug c++/91382] Missing pedwarn when using [[maybe_unused]] in C++14

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91382

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #1)
> Aren't unknown attributes supposed to be ignored?

Bug 86368 is related to that point.