Passing a descriptor as a `const Descriptor &` or a `const Descriptor *`
generates a FIR signature where the box is passed by value.
This is an issue, as it requires a load of the box to be passed. But
since, ultimately, all boxes are passed by reference a temporary is
generated in LLVM and the reference to the temporary is passed.
The boxes addresses are registered with the CUDA runtime but the
temporaries are not, thus preventing the runtime to properly map a host
side address to its device side counterpart.
To address this issue, this PR changes the signatures to the transfer
functions to pass a descriptor as a `Descriptor *`, which will in turn
generate a FIR signature with that takes a box reference as an argument.
For consistency with other dialects and other CUF passes and files, this
patch renames passes CufOpConversion to CUFOpConversion,
CufImplicitDeviceGlobal to CUFDeviceGlobal.
It also renames the file.
