clang-format: Make it possible to (require 'clang-format)

This makes the emacs integration ``provide`` a clang-format feature,
so that a .emacs can ``require`` it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221330 91177308-0d34-0410-b5e6-96231b3b80d8