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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> @arsenm Are you suggesting that these should instead be a range of minimum/maximum number of workitems globally?

That's how all of the other attributes we already have do this. amdgpu-waves-per-eu is a single min, max pair. Same with amdgpu-flat-work-group-size

Although this one is weird because it's dimensional. I'm also wondering what the use of the minimum dispatch size would be? I can see some minimal use for the maximum 

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