IntrusivePtr: replace the "add_ref" parameter with tag structs

Using a runtime parameter is obscure and error-prone.  Avoiding
error-prone code and getting reference counting right is the whole
point of this class.

src/Stmt.cc

@@ -207,7 +207,7 @@ static IntrusivePtr<EnumVal> lookup_enum_val(const char* module_name, const char
 
 	int index = et->Lookup(module_name, name);
 	assert(index >= 0);
-	IntrusivePtr<EnumVal> rval{et->GetVal(index), false};
+	IntrusivePtr<EnumVal> rval{AdoptRef{}, et->GetVal(index)};
 
 	return rval;
 	}