Fix GH-11406: segfault with unpacking and magic method closure - #11417

The magic method trampoline closure may be variadic. However, the
arg_info for the variadic argument was not set, resulting in a crash
both in reflection and in the VM.

Fix it by creating an arg_info containing a single element in case of
the variadic case. The variadic argument is the last one (and in this
case only one) in the arg_info array.

We make sure the argument info is equivalent to the argument info of
`$closure` of the following code snippet:
```
function foo(...$arguments) {}
$closure = foo(...);
```