The session dictionary attached to a Python interpeter holds variables the...

The session dictionary attached to a Python interpeter holds variables the user creates in the script interpreter
This can include objects that have complex state and need to be torn down intelligently (e.g. our SB* objects)

This will fail if the Python interpreter does not hold a valid thread state. So, acquire one, delete the session dictionary, and then let go of it on destruction

This fixes rdar://20960843

llvm-svn: 242745