[clang] [llvm] [AMDGPU] Adding the amdgpu-num-work-groups function attribute (PR #79035)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Needs documentation in AMDGPUUsage. Should also clarify behavior of 0 

https://github.com/llvm/llvm-project/pull/79035