[php-src] Issue #22324: ext/reflection: ReflectionParameter fails to load function with leading \

Issue: https://github.com/php/php-src/issues/22324
Author: jorgsowa

### Description

The following code:

```php
<?php
function demo(string $arg) {}

$p = new ReflectionParameter("\\demo", 0);
var_dump($p->getName());

$p = new ReflectionParameter("demo", 0);
var_dump($p->getName());
```

Resulted in this output:
```
Fatal error: Uncaught ReflectionException: Function \demo() does not exist in /in/a4prG:5
Stack trace:
#0 /in/a4prG(5): ReflectionParameter->__construct('\\demo', 0)
#1 {main}
  thrown in /in/a4prG on line 5
```

But I expected this output instead:
```
string(3) "arg"
string(3) "arg"
```


### PHP Version

```plain
master
```

### Operating System

_No response_