[libc++] Fix a test failure in 7b00e9fae3 (D93815).

"LLVM Buildbot on libcxx-libcxxabi-libunwind-armv7-linux" is not happy
with comparing `unsigned` and `int` [-Werror,-Wsign-compare].

GitOrigin-RevId: c0a2d3b90b3b024247cb38f73219ed595e974431
1 file changed