[clang-format] Fix incorrect isspace input (NFC)

This change fixes a clang-format unit test failure introduced by [D124748](https://reviews.llvm.org/D124748). The `countLeadingWhitespace` function was calling `isspace` with values that could fall outside the valid input range. The valid input range for `isspace` is unsigned 0-255. Values outside this range produce undefined behavior, which on Windows manifests as an assertion being raised in the debug runtime libraries. `countLeadingWhitespace` was calling `isspace` with a signed char that could produce a negative value if the underlying byte's value was 128 or above, which can happen for non-ASCII encodings. The fix is to use `StringRef`'s `bytes_begin` and `bytes_end` iterators to read the values as unsigned chars instead.

This bug can be reproduced by building the `check-clang-unit` target with a DEBUG configuration under Windows. This change is already covered by existing unit tests.

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D128786