clang-format: Don't exit with failure on empty files.

Also let clang-format-diff.py detect errors based on clang-format's
return code. Otherwise messages like "Can't find usable .clang-format,
falling back to LLVM style" can make it fail, which might be undesired.

Patch by Alp Toker. Thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192184 91177308-0d34-0410-b5e6-96231b3b80d8