[Clang] fix -Wvoid-ptr-dereference for gnu89

Follow up to D134702; it looks like we were still warning for gnu89
mode.

Link: https://reviews.llvm.org/D134702
Link: https://github.com/ClangBuiltLinux/linux/issues/1720#issuecomment-1265738778

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D135090