While we do not have yet LLVM 13 in the gitlab-ci, I use it on my
systems. So fix the new warnings found with clang-tidy 13.

Generate a new compile_commands database in compile_commands_wo_subprojects/
without the files in subprojects/.
This new compile_commands database is than consumed by iwyu and clang-tidy.

Co-Authored-By: Florian Schmaus <schmaus@cs.fau.de>
Suggested--By: Florian Schmaus <schmaus@cs.fau.de>

This matches now all headers in the repository, instead of just the
ones in the emper/ subdirectory, for example the
/tests/fixtures.network.hpp header.