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ARROW-6701: [C++][R] Lint failing on R cpp code

Closes #5514 from nealrichardson/fix-lint and squashes the following commits:

ea8b8e762 <Neal Richardson> Note that clang-format-7 is required
f83f0e193 <Neal Richardson> Incorporate @kou's suggestion
6bcc06395 <Neal Richardson> Note how to use lint.sh and have it look for clang-format rather than hard-code its location

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>