Add duplicate pragma (microsoft#6732)

Internal build that has DXC as a submodule and that is built with a
different VC toolset version started failing after the pragma got moved
up in commit 0b9acdb. Adding a duplicate pragma back at the original
location makes both compiler versions happy.

(cherry picked from commit 49d1ae9)

projects/dxilconv/include/ShaderBinary/ShaderBinary.h

@@ -383,6 +383,7 @@ class COperandBase {
   struct {
     D3D10_SB_OPERAND_INDEX_REPRESENTATION m_IndexType[3];
     D3D10_SB_OPERAND_INDEX_DIMENSION m_IndexDimension;
+#pragma warning(suppress : 4201) // Warning about nameless structure.
   };
 
   friend class CShaderAsm;