[flang][cuda] Add proper TODO for cuda fortran assignment (#85705)

Data transfer between host and device can be done with assignment
statements in CUDA Fortran. This is currently not lowered so adding a
proper TODO.


https://docs.nvidia.com/hpc-sdk/archive/24.3/compilers/cuda-fortran-prog-guide/index.html#cfref-data-trans-assgn-statemts