[libc++] Fix the check-format job

[ldionne]

Summary

Previously, it wouldn't take into account files in ignore_format.txt
(at least not on OSX) because the find command would return file names
like libcxx/src//new_handler.cpp, which never matched the file names
in ignore_format.txt.

[Mordante]

LGTM!

libcxx/utils/ci/run-buildbot

check-format)
     # We need to set --extensions so that clang-format checks extensionless files.
     mkdir -p ${BUILD_DIR}
     if [ -z "${GIT_CLANG_FORMAT}" ]; then
        GIT_CLANG_FORMAT=git-clang-format
     fi
     ${GIT_CLANG_FORMAT} \
         --diff \
         --extensions ',h,hpp,c,cpp,cppm,inc,ipp' HEAD~1 \
-        -- $(find libcxx/{benchmarks,include,modules,src}/ -type f | grep -vf libcxx/utils/data/ignore_format.txt) \
+        -- $(find libcxx/{benchmarks,include,modules,src} -type f | grep -vf libcxx/utils/data/ignore_format.txt) \
         | tee ${BUILD_DIR}/clang-format.patch
     # Check if the diff is empty, fail otherwise.
     ! grep -q '^--- a' ${BUILD_DIR}/clang-format.patch
 ;;
 check-generated-output)
     # `! foo` doesn't work properly with `set -e`, use `! foo || false` instead.
     # https://stackoverflow.com/questions/57681955/set-e-does-not-respect-logical-not
     clean