clang-format: Better support and testing for protocol buffers.

With this patch, there is dedicated testing for protocol buffers
(https://developers.google.com/protocol-buffers/).

Also some minor tweaks formatting tweaks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199580 91177308-0d34-0410-b5e6-96231b3b80d8