[flang][cuda] Add restriction on assumed size device variable (#87664)

According to
https://docs.nvidia.com/hpc-sdk/compilers/cuda-fortran-prog-guide/#cfpg-var-qual-attr-device

> A device array may be an explicit-shape array, an allocatable array,
or an assumed-shape dummy array.

Assumed size array are not supported. This patch adds an error for that
case.