It would be nice if the repository contained a clang-format
config file so that PR submitters could use tooling to correctly
format their code match Googles/abseils style.

This also makes it easier for reviewers to specify what coding style
should be used. ie the one the tooling produces.

The clang-format file is the default Google configuration as dumped by `clang-format`.
I'm not sure abseil will want the exact same options, but it seemed like the right
starting place.