DRTVWR-558: Fix LLEventDispatcher::addMethod() for LazyEventAPI.

addMethod() was using dynamic_cast<target class*>(this) and testing for
nullptr to decide whether the class owning the passed method is, or is not, a
subclass of LLEventDispatcher. The trouble is that it doesn't work for the
deferred add() calls queued by LazyEventAPI: the dynamic_cast was always
returning nullptr. static_cast works better, but that leaves us with the
problem we were trying to solve with dynamic_cast: what if the target class
really isn't a subclass? Use std::is_base_of to pick which of two addMethod()
overloads to invoke, one of which calls addFail().

(cherry picked from commit a4d520aa5d023d80cfeec4f40c3464b54cbcfc5b)