[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/None/ScriptInterpreterNone.cpp

@@ -20,6 +20,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+LLDB_PLUGIN(ScriptInterpreterNone);
+
 ScriptInterpreterNone::ScriptInterpreterNone(Debugger &debugger)
     : ScriptInterpreter(debugger, eScriptLanguageNone) {}