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

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DeanSturtevant1 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.

This change should in no way change under what conditions the diagnostics are emitted.

I included release notes. PTAL.



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