OK, victory! It took messing around with packages, but what I did was:
- Replace lib32-vulkan-radeon with lib32-vulkan-radeon-git. This removed one of the two dependency problems keeping me from downgrading to mesa-git/llvm-libs-git
- force the downgrade with
sudo pacman -Sdd mesa-git/llvm-libs-git.
The only thing I've done so far is fire up Steam and see that it works, so I'm not sure if other problems will show up with the potentially broken dependency for openmp, or if it will just happily use mesa-git/llvm-libs-git. But I'm cautiously optimistic.