Cycles already treats denoising fairly separate in its code, with a
dedicated `Denoiser` base class used to describe denoising
behavior. That class has been fully implemented for OIDN
(`denoiser_oidn.cpp`), but for OptiX was mostly empty
(`denoiser_optix.cpp`) and denoising was instead implemented in
the OptiX device. That meant denoising code was split over various
files and directories, making it a bit awkward to work with. This
patch moves the OptiX denoising implementation into the existing
`OptiXDenoiser` class, so that everything is in one place. There are
no functional changes, code has been mostly moved as-is. To
retain support for potential other denoiser implementations based
on a GPU device in the future, the `DeviceDenoiser` base class was
kept and slightly extended (and its file renamed to
`denoiser_gpu.cpp` to follow similar naming rules as
`path_trace_work_*.cpp`).
Differential Revision: https://developer.blender.org/D16502
