Fix gen_stub.php errors (#14335)

Author: vudaltsov

build/gen_stub.php

@@ -990,7 +990,7 @@ public function __construct(Name $class, string $property)
         $this->property = $property;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->class->toString() . "::$" . $this->property;
     }
@@ -1624,7 +1624,7 @@ function (Expr $expr) use ($allConstInfos, &$constType, &$originatingConst, &$is
                     return null;
                 }
 
-                throw new Exception("Constant " . $originatingConstName->__toString() . " cannot be found");
+                throw new Exception("Constant " . $constName . " cannot be found");
             }
         );