[llvm] [AMDGPU] Re-enable closed-world assumption as an opt-in feature (PR #115371)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

Okay, we can possibly make this the option passed to the backend job for `-fno-rdc` in HIP / CUDA.

https://github.com/llvm/llvm-project/pull/115371