[caffe2] use Clang identification macro in various places (#33574)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33574
Sprinkle with Clang identification macro places that otherwise would cause build errors when Clang is used to drive the CUDA compilation.
Note: `__clang__` is defined when either Clang is used as host compiler by NVCC or when Clang drives the compilation. `__CUDA__` is defined only for the latter case.
Test Plan:
```lang=bash
buck build mode/opt -c fbcode.cuda_use_clang=true //fblearner/flow/projects/dper:workflow
buck build mode/opt //fblearner/flow/projects/dper:workflow
```
Reviewed By: BIT-silence
Differential Revision: D20007440
fbshipit-source-id: 53caa70695b99461a3910d41dc71a9f6d0728a75
