[Bug c++/86368] an unknown [[attribute]] should not trigger a warning in C++17

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86368

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Justin Bassett from comment #7)
> and it won't extend to future standardized attributes.

That's a Good Thing. If I use a new standardized attribute like
[[no_unique_address]] I definitely want to know if the compiler doesn't
recognize it, because that affects the ABI of my code.