Review Request 67869: Add use of `override` to the Mesos C++ style guide.

[James Peach]

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https://reviews.apache.org/r/67869/
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Review request for mesos, Andrew Schwartzmeyer, Benjamin Bannier, Greg Mann, 
Mesos Reviewbot, Till Toenshoff, and Zhitao Li.


Bugs: MESOS-9065
    https://issues.apache.org/jira/browse/MESOS-9065


Repository: mesos


Description
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Add an explicit guideline to the Mesos C++ style guide that the use of
`override` keyword is required. Update cpplint and clang-tidy tooling
to encourage this.


Diffs
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  docs/c++-style-guide.md 80a1af318984d47bdfa641aa12e28963d3183fea 
  support/clang-tidy fcffb39089573a708dc3f2052f639a1a3621040d 
  support/mesos-style.py 27ed553cb1d9e0c3c750b414eafe0144c3442c43 


Diff: https://reviews.apache.org/r/67869/diff/1/


Testing
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Manual. The `cpplint` change doesn't appear to enforce the use of `override`.


Thanks,

James Peach