clang-format: Give clang-format-diff.py a -v option.

With it, it prints the file being formatted. Apparently people are
formatting thousands of files and some progress indication is helpful.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221990 91177308-0d34-0410-b5e6-96231b3b80d8