Fix crash when foreach key isn't a variable

[phil-nelson]

This fixes a crash when a foreach key is not a straight up variable (which I hadn't realised was possible), e.g. this is valid:

<?php

$arr = [1,2,3];
$a = new stdClass();
foreach ($arr as $a->key => $value) {
    echo "{$a->key}: $value\n";
}

Fixes #563

[codecov]

Codecov Report

Merging #564 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #564      +/-   ##
============================================
+ Coverage     81.46%   81.52%   +0.05%     
  Complexity      901      901              
============================================
  Files            61       61              
  Lines          2061     2062       +1     
============================================
+ Hits           1679     1681       +2     
+ Misses          382      381       -1

Impacted Files	Coverage Δ	Complexity Δ
src/DefinitionResolver.php	87.03% <100%> (+0.02%)	321 <0> (ø)	⬇️
src/Cache/FileSystemCache.php	25% <0%> (+5%)	8% <0%> (ø)	⬇️