Revise C++ coding style using clang-format-6.0

Run the `clang-format.bash` script to update all our C and C++ code to a
new style defined by `.clang-format`.  Use `clang-format` version 6.0.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.