ProxyGenerator::isShortIdentifierGetter should check for file existance before reading the file

We are using a module which is generating entity classes at runtime with Zend\Code\Generator which are only eval()'d and not persisted to the file system. When calling `doctrine-module orm:generate:proxies` lots of warnings are issued because the class files do not exist. Therefore, ProxyGenerator::isShortIdentifierGetter should check existance of class files before reading them with file().

Simply adding `&& file_exists($method->getDeclaringClass()->getFileName())` to the initialization of `$cheapCheck` does prevent those warnings.

Start at ProxyGenerator::isShortIdentifierGetter and inspect how $cheapCheck obtains the declaring class file before calling file(). Reproduce with doctrine-module orm:generate:proxies using runtime-generated, non-persisted entity classes, then confirm the command completes without warnings for missing class files.

Written by the indexing model from the issue text.