Setup Include What You Use (IWYU)

[jrhemstad]

To ensure CCCL's headers are as slim as possible, we should setup the IWYU tool as part of our devcontainer/CI workflow.

[fbusato]

I did my own research related to IWYU for my course. It is hard to integrate because it requires a non-standard LLVM version. Second, not sure if CUDA is supported.
Here the slides that highlight the problem