lazyTypeFilterTree

If this message bundle tree has a type filter tree, return it, otherwise null.

The type filter tree is used to quickly eliminate potential bundle invocations based on the type of an argument that has just been parsed. The argument's expression type is looked up in the tree, and the result is which bundle tree should be visited next, having eliminated all parsing possibilities where the argument was of an unacceptable type.

This is only authoritative if an expand has been invoked since the last modification via methods like addPlanInProgress.