clang-format.bash: update to clang-format-6.0
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.
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