[php-src] Issue #14738: Make ::class keyword returns ReflectionClass object and add ReflectionClass->__toString() for backward compatibility

Issue: https://github.com/php/php-src/issues/14738 Author: coderapers Make ::class keyword returns ReflectionClass object and add ReflectionClass->__toString() for backward compatibility. It's needed for show us that functions, whoose really get class name as argument. Example: ```php function wrongFunctionDescriptor(string $className) { /* ... */ } ``` and it will be correct as ```php function correctFunctionDescriptor(\ReflectionClass $className) { /* ... */ } ``` Because I can send to function wrongFunctionDescriptor() any string, like "FOO-BAR BUZ!" and it will not correctly class name. When function correctFunctionDescriptor() got ONLY class names, returned by ReflectionClass->__toString() as full class name.