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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-debian-cpp20` running on `clang-debian-cpp20` while building `llvm` at step 2 "checkout".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/108/builds/6971

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 2 (checkout) failure: update (failure)
git version 2.43.0
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': Could not resolve host: github.com

```

</details>

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