Surrogate TUs are TUs that only include one single header, and do nothing else. These TU must still include: if they do not, then a header doesn't stand on its own, and there is likely some include or declaration missing from it.

There are three parts to this

• Add surrogate TUs for all current headers
• Ensure that they are built during testing (probably under "Extra Tests" CMake toggle, to save compilation times for common builds)
• Add CI script that checks whether all headers have their surrogate TU