Clean up ggml-cuda.cu warnings when compiling with clang (for ROCM) (#4124)

* ggml-cuda.cu: Clean up warnings when compiling with clang

* ggml-cuda.cu: Move static items into anonymous namespace

* ggml-cuda.cu: Fix use of namespace start macro

* Revert "ggml-cuda.cu: Fix use of namespace start macro"

This reverts commit 26c1149.

* Revert "ggml-cuda.cu: Move static items into anonymous namespace"

This reverts commit e29757e.

Author: KerfuffleV2