[Python] Remove Python include from ScriptInterpreterPython.h

This patch limits the scope of the python header to the implementation
of the python script interpreter plugin. ScriptInterpreterPython is now
an abstract interface that doesn't expose any Python specific types, and
is implemented by the ScriptInterpreterPythonImpl.

Differential revision: https://reviews.llvm.org/D59976

llvm-svn: 357307