Merge pull request #5112 from YosysHQ/krys/on_shutdown

design.cc: Use on_shutdown method

kernel/register.h

@@ -29,6 +29,7 @@ struct Pass
 {
 	std::string pass_name, short_help;
 	Pass(std::string name, std::string short_help = "** document me **");
+	// Prefer overriding 'Pass::on_shutdown()' if possible
 	virtual ~Pass();
 
 	virtual void help();

passes/cmds/design.cc

@@ -28,7 +28,7 @@ std::vector<RTLIL::Design*> pushed_designs;
 
 struct DesignPass : public Pass {
 	DesignPass() : Pass("design", "save, restore and reset current design") { }
-	~DesignPass() override {
+	void on_shutdown() override {
 		for (auto &it : saved_designs)
 			delete it.second;
 		saved_designs.clear();