[clang] [HLSL] Implement D3DCOLORtoUBYTE4 intrinsic (PR #122202)

[Justin Bogner via cfe-commits]

================
@@ -41,6 +41,14 @@ constexpr enable_if_t<sizeof(U) == sizeof(T), U> bit_cast(T F) {
   return __builtin_bit_cast(U, F);
 }
 
+constexpr vector<uint, 4> d3d_color_to_ubyte4_impl(vector<float, 4> V) {
+  // Use the same scaling factor used by FXC (i.e., 255.001953)
+  // Excerpt from stackoverflow discussion:
+  // "Built-in rounding, necessary because of truncation. 0.001953 * 256 = 0.5"
+  // https://stackoverflow.com/questions/52103720/why-does-d3dcolortoubyte4-multiplies-components-by-255-001953f
----------------
bogner wrote:

It might be better to refer to the implementation in DXC here, rather than copying the comment in DXC about compatibility with FXC.

https://github.com/llvm/llvm-project/pull/122202