[clang] [llvm] [HLSL][SPIR-V] Implements SV_Position for VS/PS I/O (PR #168735)

[Steven Perron via cfe-commits]

================
@@ -961,11 +967,14 @@ void SemaHLSL::checkSemanticAnnotation(
   }
 
   if (SemanticName == "SV_POSITION") {
-    // TODO(#143523): allow use on other shader types & output once the overall
-    // semantic logic is implemented.
-    if (ST == llvm::Triple::Pixel)
+    // SV_Position can is I/O for vertex shaders.
+    // For pixel shaders, only valid as input.
----------------
s-perron wrote:

```suggestion
    // SV_Position can be an input or output in vertex shaders,
    // but only an input in pixel shaders.
```

https://github.com/llvm/llvm-project/pull/168735