Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before. Then revise the style:
* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`
Rename the Git attribute identifying the format to include the
clang-format version number: `format.clang-format-6.0`. This will aid
external infrastructure in knowing what version of the tool to run.