[FEA]: Diff libcudacxx codegen output against header in repo

### Is this a duplicate?

- [X] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this request and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Area

libcu++

### Is your feature request related to a problem? Please describe.

Currently we have no way to validate that the codegen.cpp and generated atomic header are in the same state. We should configure and build codegen in a small GHA to confirm that the atomic generated header has not been modified by hand.

### Describe the solution you'd like

A small GHA can be spun up that builds codegen, (it has no special requirements), and test that the output of it matches the atomic generated header in repo.

### Describe alternatives you've considered

_No response_

### Additional context

#478 makes changes to codegen and the atomic generated header.