[Bug c/107947] __has_c_attribute incorrectly identifies attribute as supported

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107947

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to WANG Xuerui from comment #0)
> I haven't bisected yet but can help doing so if necessary.

Bisection shows that it changed with r13-2976 which is the commit that added
support for the [[noreturn]] attribute. So it's totally correct that
__has_c_attribute starts to be non-zero then, because it started to be
supported then.

    c: C2x noreturn attribute

    C2x adds a standard [[noreturn]] attribute (which can also be spelt
    [[_Noreturn]] for use with <stdnoreturn.h>), so allowing non-returning
    functions to be declared in a manner compatible with C++; the
    _Noreturn function specifier remains available but is marked
    obsolescent.