Description:
------------
If property is assigned a default string value in the form of heredoc, its ReflectionProperty::getDocComment() returns false even if there's a PHPDoc.

Looks like this bug was introduced in PHP 7.3.0: https://3v4l.org/4fRLA

Related PHPStan bug: https://github.com/phpstan/phpstan/issues/2742

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

class BugReportMailrcConfigTests
{
    /** @var string */
    private $s1 = <<<STRING
I'm a string :(
STRING;
    /** @var string */
    private $s2 = <<<'STRING'
I'm a string :)
STRING;
    /** @var string */
    private $s3 = 'I\'m a string :)';
}

$ref = new \ReflectionClass(BugReportMailrcConfigTests::class);
$s1 = $ref->getProperty('s1');
var_dump($s1->getDocComment());

$s2 = $ref->getProperty('s2');
var_dump($s2->getDocComment());

$s3 = $ref->getProperty('s3');
var_dump($s2->getDocComment());

Expected result:
----------------
string(18) "/** @var string */"
string(18) "/** @var string */"
string(18) "/** @var string */"

Actual result:
--------------
bool(false)
string(18) "/** @var string */"
string(18) "/** @var string */"