hip-clang enablement (#16085)

hip-clang enablement (#16085)

Summary:
Initial enabling of the upcoming hip-clang compiler for the PyTorch source base.

Changes:
* update the Eigen submodule to a version including our upstreamed hip-clang enabling there
* modify a few ifdef guards with the `__HIP__` macro used by hip-clang
* use `__lane_id` instead of `hc::__lane_id`
* add Debug flags for ROCm to the cmake infrastructure
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16085

Differential Revision: D13709459

Pulled By: ezyang

fbshipit-source-id: 1b7b33fe810a0434766180580d4443ea177eb7c7