This change fixes a clang-format unit test failure introduced by 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.