Previously the class `ExplodedGraph` had a data member called `Roots`
which could (in theory) store multiple root nodes -- but in practice
exploded graphs always had at most one root node (zero was possible for
empty and partially copied graphs) and introducing a graph with multiple
roots would've caused severe malfuncitons (e.g. in code that used the
pattern `*roots_begin()` to refer to _the_ root node).
I don't see any practical use case for adding multiple root nodes in a
single graph (this seems to be yet another of the "generalize for the
sake of generalization" decisions which were common in the early history
of the analyzer), so this commit replaces the vector `Roots` with
`ExplodedNode *Root` (which may be null when the graph is empty or under
construction).
Note that the complicated logic of `ExplodedGraph::trim` deserves a
through cleanup, but I left that for a follow-up commit.
31e981ca1d