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

[Victor Chernyakin via cfe-commits]

localspook wrote:

For the encoding issue: could you try adding
```python
sys.stdout.reconfigure(encoding='utf-8')
sys.stderr.reconfigure(encoding='utf-8')
```
to `main` in `check_clang_tidy.py`, then see if that fixes CI? I'm not sure if it'll work, since the issue doesn't seem to reproduce on my Windows setup, but I would like to try and avoid disabling the tests

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