Configure a basic .clang-tidy.

Also, fix some issues that it found.

We will enable more rules in the future. In particular, some google-*
rules should already pass, but appear not to at the moment.

Once we have enabled enough of these rules, we intend to drop the
dependency on cpplint.

Change-Id: I107231cf57b5500a3f1b623510419665589531dc