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

[via cfe-commits]

github-actions[bot] wrote:

<!--LLVM CODE LINT COMMENT: clang-tidy-->


:warning: C/C++ code linter clang-tidy found issues in your code. :warning:

<details>
<summary>
You can test this locally with the following command:
</summary>

```bash

git diff -U0 origin/main...HEAD -- clang-tools-extra/clang-tidy/utils/FormatStringConverter.cpp |
python3 clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py \
  -path build -p1 -quiet
```

</details>

<details>
<summary>
View the output from clang-tidy here.
</summary>

```
clang-tools-extra/clang-tidy/utils/FormatStringConverter.cpp:703:11: warning: use auto when initializing with a cast to avoid duplicating the type name [modernize-use-auto]
  703 |     const unsigned char UCh = static_cast<unsigned char>(Ch);
      |           ^~~~~~~~~~~~~
      |           auto
```

</details>


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