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

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 26c11490266c096e3e5731e05270a8f73a5b2874.

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

This reverts commit e29757e0f7535d1ac314300f0324684cc785e06c.