Update clang format to 8

The main reason to update from 7 to 8 is, for me, the fix to bug https://bugs.llvm.org/show_bug.cgi?id=35641

Apart from that, the new version only makes minor changes to the current version, which, I think, improve readability.

Ubuntu 18.04 users, clang-8 can be installed via the binary tarball from http://releases.llvm.org/download.html, while CentOS 7 users can pick up the SFT build in /cvmfs/sft.cern.ch/lcg/releases/clang/8.0.0-ed577