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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> > Seeing link errors from `CustomModuleAnchorSource`
> 
> I think this was likely a CI issue? It seems that the build passes now: https://lab.llvm.org/buildbot/#/builders/174/builds/28167

Yeah, while trying to regress that cause I'm seeing build issues implying something stale is not being rebuilt

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