[clang] [llvm] [HLSL][SPIRV][DXIL] Implement `dot4add_i8packed` intrinsic (PR #113623)

[Finn Plummer via cfe-commits]

================
@@ -894,6 +894,16 @@ uint64_t dot(uint64_t3, uint64_t3);
 _HLSL_BUILTIN_ALIAS(__builtin_hlsl_dot)
 uint64_t dot(uint64_t4, uint64_t4);
 
+//===----------------------------------------------------------------------===//
+// dot4add builtins
+//===----------------------------------------------------------------------===//
+
+/// \fn int dot4add_i8packed(uint A, uint B, int C)
+
+_HLSL_16BIT_AVAILABILITY(shadermodel, 6.4)
----------------
inbelic wrote:

```suggestion
_HLSL_AVAILABILITY(shadermodel, 6.4)
```
Whoops, will update with other review comments.

https://github.com/llvm/llvm-project/pull/113623