[clang] [clang] Warn on signed char array constant conversion (PR #203792)

[Aaron Ballman via cfe-commits]

================
@@ -646,6 +646,8 @@ Improvements to Clang's diagnostics
 - Diagnostics for the C++11 range-based for statement now report the correct
   iterator type in notes for invalid iterator types.
 
+- Fixed a missing ``-Wconstant-conversion`` diagnostic for ``signed char`` array initialization.
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AaronBallman wrote:

```suggestion
- Fixed a missing ``-Wconstant-conversion`` diagnostic for ``signed char`` array initialization. (#GH181730)
```

https://github.com/llvm/llvm-project/pull/203792