[libc-commits] [libc] [libc] Enable `-Wunused` and clean up found instances (PR #96949)

[Michael Jones via libc-commits]

michaelrj-google wrote:

I think `-Wunused` is included in `-Wall` and `-Wextra`, so closing this PR and issue SGTM.

https://github.com/llvm/llvm-project/pull/96949