Fix some compilation warnings.

Redone the commit based on the input in
https://github.com/godotengine/godot/pull/7851 . Not all warnings were
fixed but it's a start.

modules/gdscript/gd_editor.cpp

@@ -2645,6 +2645,7 @@ Error GDScriptLanguage::lookup_code(const String& p_code, const String& p_symbol
 
 	switch(p.get_completion_type()) {
 
+		case GDParser::COMPLETION_GET_NODE:
 		case GDParser::COMPLETION_NONE: {
 		} break;
 		case GDParser::COMPLETION_BUILT_IN_TYPE_CONSTANT: {