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

[via cfe-commits]

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TPPPP72 wrote:

Looking at the original code `if (CC.isValid() && T->isCharType())`, the type check was already performed upfront.
We should keep the cheap `T->isCharType()` (and the new SChar check) at the very beginning to short-circuit

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