Add Unknown/NoShadow activity

In a session with @boriskaus, I had the issue that I wasn't sure if a gradient was zero, because it was zero, -- or because it was propagated through Memory annotated as `Const`. IIUC user provided annotation are strong and as such Enzyme will do as the user said.

For the default in Enzyme.jl I would like to say: No information provided, if you need to store gradient information to the shadow of this input, throw an error and the user must annotate it either Const or Duplicated