[clang] [HLSL][SPIR-V] Handle SV_Postion builtin in PS (PR #141759)

[Farzon Lotfi via cfe-commits]

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@@ -8472,6 +8472,20 @@ The full documentation is available here: https://docs.microsoft.com/en-us/windo
   }];
 }
 
+def HLSLSV_PositionDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``SV_Position`` semantic, when applied to an input parameter in a pixel
+shader, contains the location of the pixel center (x, y) in screen space.
+This semantic can be applied to the parameter, or a field in a struct used
+as input parameter.
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farzonl wrote:

```suggestion
as an input parameter.
```

https://github.com/llvm/llvm-project/pull/141759