I just merged ARROW-3972 https://github.com/apache/arrow/commit/c81fbaa20ce4d14ae08bbb2f4a959778f1069d83 with Ravindra's and Uwe's help which upgrades the codebase to LLVM 7. You will want to upgrade your clang, LLVM, and clang-format for C++ development.
Luckily it seems that the behavior of clang-format didn't change from LLVM 6 to LLVM 7 so with luck there will be fewer linting failure during this transition. - Wes
