incr.comp.: Do per-MonoItem dependency tracking in order to collect data about granularity fallout.

Currently the compiler only allocates a single DepNode per CGU. This is enough for implementing object file re-use but we are losing information about what items within an object file exactly caused it to be re-compiled. In order to gather more information about how CGU partitioning affects re-use, it would be good to know which MonoItems within a CGU were changed and which were not.

Before making this the default though, we should check how it affects the number of DepNodes and edges in the graph.