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

[Erich Keane via cfe-commits]

================
@@ -2705,6 +2705,30 @@ An error will be given if:
   }];
 }
 
+def AMDGPUNumWorkGroupsDocs : Documentation {
+  let Category = DocCatAMDGPUAttributes;
+  let Content = [{
+The number of work groups specifies the number of work groups when the kernel
+is dispatched.
+
+Clang supports the
+``__attribute__((amdgpu_num_work_groups(<x>, <y>, <z>)))`` attribute for the
+AMDGPU target. This attribute may be attached to a kernel function definition
+and is an optimization hint.
+
+``<x>`` parameter specifies the maximum number of work groups in the x dimentsion.
----------------
erichkeane wrote:

```suggestion
``<x>`` parameter specifies the maximum number of work groups in the x dimension.
```

Also, we should be more clear/elaborate more what `x`, `y`, and `z` dimensions mean here. One thing I note is that `OpenCL` (IIRC?) actually reverses these?  So it is VERY important that we document both order and meaning explicitly.

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