[clang-tools-extra] [clang-tidy] Fix OOB access in `FormatStringConverter` with signed chars (PR #169215)

[via cfe-commits]

zeyi2 wrote:

> ```python
> sys.stdout.reconfigure(encoding='utf-8')
> sys.stderr.reconfigure(encoding='utf-8')
> ```

Thanks for the suggestion! The Windows build now passes :)


https://github.com/llvm/llvm-project/pull/169215