To cover STR34-C rule's second use case, where `signed char` is
used for array subscript after an integer conversion. In the case
of non-ASCII character this conversion will result in a value
in excess of UCHAR_MAX.
There is another clang-tidy check which catches these cases.
cppcoreguidelines-pro-bounds-constant-array-index catches any
indexing which is not integer constant. I think this check is
very strict about the index (e.g. constant), so it's still useful
to cover the `signed char` use case in this check, so we
can provide a way to catch the SEI cert rule's use cases on a
codebase, where this CPP guideline is not used.