VecGeom Clang Fix, main branch (2023.10.23.)

I am not a happy camper by now... 😦 Making this work ended up being a lot more complicated than I first thought. 😦

Unfortunately the CMake configuration of both VecCore and VecGeom is khm, questionable... The configurations try to be super smart in some things, while doing very stupid things in others. 🤔

I'll give a more detailed explanation in MRs going into those two projects, later in the week. But both of their build configurations will need some serious improvements. Until then, this hack should allow us to progress on ATLINFR-5113.