[clang] [sema] Improve -Wsign-compare (PR #65684)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> Yes, there's this way and also when you compare integers smaller than `int`, but those don't trigger -Wsign-compare. -Wsign-compare only triggers if a signed operand is converted to an unsigned operand. This is a PR description bug :)

I did not understand that. I can see why the unsigned case is more problematic but I don't know if I like the asymmetry in the diagnostic. The singed case can still result in really confusing results.  

https://github.com/llvm/llvm-project/pull/65684