Fix a race condition where multiple PythonInputReaderManager instances could, during destruction,

tread on the m_embedded_thread_input_reader_sp singleton maintained by the script interpreter.
Furthermore, use two additional slots under the script interpreter to store the PseudoTerminal and
the InputReaderSP pertaining to the embedded python interpreter -- resulted from the
ScriptInterpreterPython::ExecuteInterpreterLoop() call -- to facilitate separation from what is being
used by the PythonInputReaderManager instances.

llvm-svn: 162147