adding comment

Author: joaosaffran

llvm/lib/Target/DirectX/DXILRootSignature.cpp

@@ -76,6 +76,8 @@ static bool parseRootConstants(LLVMContext *Ctx, mcdxbc::RootSignatureDesc &RSD,
     return reportError(Ctx, "Invalid format for RootConstants Element");
 
   dxbc::RootParameterHeader Header;
+  // this will be properly calculated when writing it.
+  Header.ParameterOffset = 0;
   Header.ParameterType =
       llvm::to_underlying(dxbc::RootParameterType::Constants32Bit);