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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).