[Bug c/89031] Should not report unused result when explicitly cast to void

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89031

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This attribute is documented this way.  I know there are a few duplicates of
this bug already filed too.