[clang] [clang] Extend -Wunused-but-set-variable to static globals (PR #178342)

[John Paul Jepko via cfe-commits]

jpjepko wrote:

I noticed there were some CI issues with file-scope volatiles that were hidden by previous failures. Unlike volatile locals, there was no precedent to follow in either other clang warnings or GCC. I decided to simply not warn on them since those writes may be observed externally (read by other threads, for example), unlike volatile locals.

https://github.com/llvm/llvm-project/pull/178342