This is the node used for our hashtable of vtable map variable information. When we create a vtable map variable (var decl) we put it into one of these nodes; create a corresponding vtv_graph_node for our class hierarchy info and store that in this node; generate a unique (monotonically ascending) id for both the vtbl_map_node and the vtv_graph_node; and insert the node into two data structures (to make it easy to find in several different ways): 1). A hash table ("vtbl_map_hash" in vtable-verify.c). This gives us an easy way to check to see if we already have a node for the vtable map variable or not; and 2). An array (vector) of vtbl_map_nodes, where the array index corresponds to the unique id of the vtbl_map_node, which gives us an easy way to use bitmaps to represent and find the vtable map nodes.
