[llvm] [clang] [AMDGPU] Adding the amdgpu-num-work-groups function attribute (PR #75647)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

How does this attribute relate to `reqd_work_group_size` and related existing attributes? Is `amdgpu_num_work_groups` the best way to expose this given that other technologies also have the notion of work groups that aren't related to AMD GPUs? CC @bader @jdoerfert 

https://github.com/llvm/llvm-project/pull/75647