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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

One attribute, with a range, would be better than two attributes. This is how it is handled in the similar cases.

I also think this should be in terms of work items, not workgroups 

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