[clang] Fix Clang bug that -Wformat-signedness is not reported properly. (PR #150962)

[Karl-Johan Karlsson via cfe-commits]

karka228 wrote:

> Note that PR [74440](https://github.com/llvm/llvm-project/pull/74440) contains a review comment that the following property would be good: "diagnostic text shows the name of the warning flag (in case you need to look it up or disable it)" That being said, this change removes the gcc-compatible point 6 which is that the diagnostics are associated with -Wformat. There was a remark about the strangeness of this, and it fact this interferes with the accuracy of work I am doing to categorize warnings that might fire in my company's codebase if enabled.

I think we all agree that the way gcc implemented this from the beginning is strange and that implied that the implementation in clang also is strange. I think I leave this up to @AaronBallman to decide if the gcc-compatible in point 6 which is that the diagnostics are associated with -Wformat, discussed in the original PR [74440](https://github.com/llvm/llvm-project/pull/74440), is important or not.

https://github.com/llvm/llvm-project/pull/150962