Use root type system, not interpreter.

A fix from Danilo Piparo:
Our AutoLibraryLoader now uses the ROOT interpreter to determine if the dictionary for a class has been loaded.
This is bad in ROOT 6 for two reasons:
1) A class with a dictionary may be known to the ROOT type system without being known to the interpreter.
2) Calling the interpreter here may invoke header parsing, which uses a significant amount of memory.
This simple ROOT6 specific change checks to see if the class is known to the ROOT type system, rather than to the interpreter.
Please merge this request as soon as convenient.