Add preliminary .clang-format file.
Our current coding style is not very uniform and we don't seem to be following any particular standard, even though there are some patterns we more or less follow. This attempts to mostly reproduce our current style. However, it still introduces quite a lot of changes since clang-format doesn't yet support dangling parentheses[*] (where the closing parenthesis of a parameter list goes on its own line). [*]: https://reviews.llvm.org/D33029