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. 
