Add clang-tidy to the CI

Borrowed from:

• https://ortogonal.github.io/cmake-clang-tidy/
• https://clang.llvm.org/extra/clang-tidy/

This will be run along with the coverity scan, on a daily basis, on the master branch. Currently, it'll only show the warnings in the job's output, but the plan™ is to take advantage of gitlab pages to show the warnings in a more accessible way.

I avoided to run clang-tidy only on changed files (like it's done here) on purpose, since I think it makes more sense to have a global cronjob instead.