Update llvmlite to 0.46.0 - prereq for updating numba for Python 3.14

Hi! Here is some updated packaging for llvmlite 0.46.0 to go into experimental. The current release requires llvm-20, so this is only a start of the process of getting llvmlite and numba updated. It would, however, be useful to have the updated package land in experimental to see buildd logs on different architectures, build numba against it, etc.

I have the upstream and pristine-tar branches to go with this also in my fork - I can push all these to the canonical repo if that is useful, or you can easily make them yourselves.

Please also let me know if you'd like me to just upload this straight to experimental.