.arcconfig

formatting: enforce clang-format on all .cc files as well as .c files

As part of my upcoming fuzzer patch we're going to grow some .cc
files. They're not part of the release build (they're only used for
fuzzing, which requires clang 6 which in turn implies a working C++
compiler), so I'm not terribly worried about growing some C++ files.

Differential Revision: https://phab.mercurial-scm.org/D1874

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