[clang] [HLSL] Implement the dst HLSL Function (PR #133828)

[Ashley Coleman via cfe-commits]

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@@ -174,6 +174,31 @@ const inline float distance(__detail::HLSL_FIXED_VECTOR<float, N> X,
                             __detail::HLSL_FIXED_VECTOR<float, N> Y) {
   return __detail::distance_vec_impl(X, Y);
 }
+//===----------------------------------------------------------------------===//
+// dst builtins
+//===----------------------------------------------------------------------===//
+
+/// \fn fvector dst( fvector, fvector)
+/// \brief Returns the length of a vector
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V-FEXrt wrote:

This description sounds off to me. Is this quoted from official documentation anywhere? To me the "length" of a `float4` is `4`.

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dst says `Returns the computed distance vector` but any official documentation is probably fine

https://github.com/llvm/llvm-project/pull/133828