Request: add support for void return type to hhvm.php7 settings (or php 7.1 settings)

[TysonAndre]

HHVM Version

nightly/all, as of today

Standalone code, or other way to reproduce the problem

With the hhvm.php7.all=1 ini setting

<?php
class MyClass {
     function foo() : void { }
}

Related to #7544 - This was filed since I didn't see any issues for this when searching

Expected result

There should be an ini setting which makes void a valid (non-class) type hint in php compatibility mode (In files with the tag <?php)
(Not aware of any settings when looking at ini documentation)

Actual result

Uncaught TypeError: Value returned from method MyClass::foo() must be of type void, null given in test.php:3