[lldb][NFC] Defer python init until ScriptInterpreter is created

Previously python was initialized during static registration of the
plugin. This causes the python interpreter to run even if python support
is explicitly disabled thru:

    SBDebugger::SetScriptLanguage(ScriptLanguage::eScriptLanguageNone)

This commit defers python initialization until a ScriptInterpreterPython
instance is created.

Author: danobi

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

@@ -352,7 +352,6 @@ void ScriptInterpreterPython::Initialize() {
                                   GetPluginDescriptionStatic(),
                                   lldb::eScriptLanguagePython,
                                   ScriptInterpreterPythonImpl::CreateInstance);
-    ScriptInterpreterPythonImpl::Initialize();
   });
 }
 
@@ -429,6 +428,10 @@ ScriptInterpreterPythonImpl::ScriptInterpreterPythonImpl(Debugger &debugger)
       m_active_io_handler(eIOHandlerNone), m_session_is_active(false),
       m_pty_secondary_is_open(false), m_valid_session(true), m_lock_count(0),
       m_command_thread_state(nullptr) {
+  static llvm::once_flag g_once_flag;
+  llvm::call_once(g_once_flag, []() {
+    ScriptInterpreterPythonImpl::Initialize();
+  });
 
   m_dictionary_name.append("_dict");
   StreamString run_string;