[lldb] [Host] Refactor TerminalState

Refactor TerminalState to make the code simpler.  Move 'struct termios'
to a PImpl-style subclass.  Add an RAII interface to automatically store
and restore the state.

Differential revision: https://reviews.llvm.org/D110721

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -355,7 +355,6 @@ private:
     PyEval_InitThreads();
   }
 
-  TerminalState m_stdin_tty_state;
   PyGILState_STATE m_gil_state = PyGILState_UNLOCKED;
   bool m_was_already_initialized = false;
 };