Cast a few vars to void that are otherwise unused
Summary:
llvm-13 marks this as an error when a variable is set but not used.
Evidently this macro doesn't always expand to using the var. Work around that
here with void casts.
Test Plan: nfc
Reviewed By: drodriguez
Differential Revision: D30062462
fbshipit-source-id: ff868ec74116da99afd539142996d2ffffd399fb
