Once SERVER-79367 is merged, we should add a const-folding step that detects when we have the following ABT:

FunctionCall("makeBsonObj", Constant [MakeObjSpec], Constant [anything], <any args that don't depend on any external variables>)

And const-folds it into a Constant [<object>] according to the given MakeObjSpec (second argument). Note that the same logic can be used for makeObj as well. One idea would be to just execute this SBE expression once and set the output to the constant value.