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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I'd like to understand the need for this diagnostic a bit more. I realize GCC has it, but it's off-by-default in GCC and `-Wformat` does not enable it either: https://godbolt.org/z/Gxczsjdj5

We believe that users don't enable off by default warnings often enough to warrant supporting them, and this feels like a very pedantic warning in many cases. e.g., `printf("%u", 0);`

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