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

[via llvm-commits]

================
@@ -918,6 +918,9 @@ def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>,
   HelpText<"Enable warnings for deprecated constructs and define __DEPRECATED">;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>,
   Visibility<[ClangOption, CC1Option]>;
+def Wformat_signedness : Flag<["-"], "Wformat-signedness">,
----------------
apple-fcloutier wrote:

There's a lot of good, free integration that you get with a real warning flag:

* -Wno symmetry (as @hazohelet already said)
* control via `#pragma clang diagnostic`
* diagnostic text shows the name of the warning flag (in case you need to look it up or disable it)

-Wformat-pedantic is another warning that "modifies" -Wformat and it exists as its own warning. I think that it would make sense to do the same thing with -Wformat-signedness.

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