Prevent void param type

[WoutervdBrink]

PHP 7.1 introduced the void type. Consequently, it is no longer allowed to specify void as the parameter type:

<?php
function foo(void $bar) {}

• This pull request adds detection for this behavior and rejects functions that have the identifier void as parameter type.
• Pre-7.1, it was allowed to create a class class void {} and use that as a parameter type. In this case, the parameter's type is the name void, so the error isn't thrown.
• Two test cases are added to verify the above behavior.

The 5.0-7.1 behavior is tested on 3v4l: https://3v4l.org/Krff3#veol

Fixes #1137.