[lldb/Plugins] Use external functions to (de)initialize plugins

This is a step towards making the initialize and terminate calls be
generated by CMake, which in turn is towards making it possible to
disable plugins at configuration time.

Differential revision: https://reviews.llvm.org/D74245

lldb/source/Plugins/ScriptInterpreter/Lua/ScriptInterpreterLua.cpp

@@ -19,6 +19,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+LLDB_PLUGIN(ScriptInterpreterLua);
+
 class IOHandlerLuaInterpreter : public IOHandlerDelegate,
                                 public IOHandlerEditline {
 public: