GN files are checked using the canned presubmit check but we cannot do
the same for C/C++ files: the canned check uses git cl format which can
only use clang-format 5.0 and requires formatting of all the source
code.
Instead we write our own clang-format check, reusing the script that
checks formatting on Travis. To have a recent version of clang-format we
import one from a helper repo.
This also fix a formatting error in .gn and adds licenses to the
clang-format linting scripts.
Change-Id: I4d8208472a8a6bd32ae3ef41c3145abf270a4c37
Corentin Wallez