[Bug objc/77428] incorrect 'set but not used' warning with @throw

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77428

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
Created attachment 39528
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39528&action=edit
tentative patch