Differentiated IR essentially built - [State 5]

The differentiated IR is accessible through a variable (e.g. diffCallGraph) of type CallGraph. It is a standalone CallGraph: it is different from the original source CallGraph, and shares very little with it. The differentiated CallGraph is ready for source regeneration.

In the current Tapenade, the only thing done with the differentiated CallGraph is source regeneration. No data-flow analysis will be run on it. Therefore we don’t need to prepare the data-flow infrastructure on the differentiated CallGraph.