Teach the extras repository to actually install the clang-format tool.

We should also install the clang-format.py script somewhere that makes
since for a Vim integration script. I don't know where that is though,
so just installing the binary for now. This is enough to let me use the
script from a checkout combined with the installed (and thus less likely
to crash or be slow) clang-format binary.

llvm-svn: 171164