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

[Erich Keane via cfe-commits]

erichkeane wrote:

Also note, this is missing Clang lit tests, and doesn't seem to be correctly handling dependent expressions for x,y, and z.

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