Investigate clang 8 weirndess
Clang 8 is giving me trouble. It's not emitting some warnings on my local computer (debian 10 + backports) but in the CI it is, see https://gitlab.dune-project.org/joe/test-deprecation/-/jobs/140512. Even more weird: when I fire up the same docker image as in the ci, and run clang++-8 manually on one of the sources that produced warnings in the ci, I don't get any warnings.