[llvm] [HLSLSemanticSignatures] Implement the optimal packing of elements (PR #218064)

[Helena Kotas via llvm-commits]

================
@@ -105,6 +105,25 @@ LLVM_ABI Expected<unsigned>
 packSignatureIndexed(MutableArrayRef<SemanticSignatureElement> Elements,
                      Triple::EnvironmentType ShaderStage, IOType IOTy);
 
+/// Packs eligible signature elements in an optimized order by reordering
+/// elements into a optimal packind order and allowing clip/cull to share
+/// comapatible rows. Only StartRow and StartCol are modified
----------------
hekota wrote:

```suggestion
/// compatible rows. Only StartRow and StartCol are modified.
```

https://github.com/llvm/llvm-project/pull/218064