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

[Shilei Tian via llvm-commits]

shiltian wrote:

I updated the PR to make it as an opt-in feature, as well as the description. FWIW, since we "support" cross-code-object global variables, we really can't assume anything _by default_ thus we can't toggle anything in the front end as well.

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