Description:
------------
Using {$this::CONSTANT} in Heredoc causes a parse error.


Test script:
---------------
<?php

class A
{
    const A = 'a';
    static private $c;
    
    public function render()
    {
        return <<<EOT
'{$this::$c}'
'{$this::A}'
EOT;
    }
}

Expected result:
----------------
{$this::A} should yield the constant value in Heredoc.

Actual result:
--------------
7.0.0 - 7.1.1: Parse error: syntax error, unexpected '}', expecting '['
5.6.0 - 5.6.30: Parse error: syntax error, unexpected '}', expecting '('