GDScript: Make check for exposed classes more consistent

Some places were already checking if classes from ClassDB were exposed,
while others didn't. This makes the check more consistent to avoid
disparities which can lead to crashes.

modules/gdscript/gdscript_analyzer.cpp

@@ -6415,7 +6415,7 @@ void GDScriptAnalyzer::resolve_pending_lambda_bodies() {
 	static_context = previous_static_context;
 }
 
-bool GDScriptAnalyzer::class_exists(const StringName &p_class) const {
+bool GDScriptAnalyzer::class_exists(const StringName &p_class) {
 	return ClassDB::class_exists(p_class) && ClassDB::is_class_exposed(p_class);
 }