[clang] [llvm] [Sema] Implement support for -Wformat-signedness (PR #74440)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Can you also add a test showing the difference between `-Wformat -Wformat-signedness` and `-Wformat-signedness` by itself (which does nothing)? I'd also like to see a test demonstrating that `#pragma GCC diagnostic ignored -Wformat` disables the signedness warnings.

Otherwise, this generally LGTM!

https://github.com/llvm/llvm-project/pull/74440