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

[via cfe-commits]

zeyi2 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

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