[flang][cuda] Add UNIFIED data attribute (#88171)

Latest version of the specification introduced the `UNIFIED` attribute
for data.


https://docs.nvidia.com/hpc-sdk/compilers/cuda-fortran-prog-guide/#cfref-var-attr-unified-data

This patch adds the attribute to parsing, semantic and lowering. 

The matching rules for dummy/actual arguments is not part of this patch.