[hack] Assigning list() should be ok with Pair
[hack] Assigning list() should be ok with Pair

Summary:You should be able to do:

```
<?hh //strict
function main(): void {
  $p = list($x, $y) = Pair {0, 1};
}
```
without erroring like:

```
a.php:4:3,39: You are using the return value of a void function (Typing[4119])
  a.php:4:8,23: A void function doesn't return a value
```

Reviewed By: int3

Differential Revision: D3113087

fb-gh-sync-id: 61fbc3896a5454927024a6bc87474026d5306971
fbshipit-source-id: 61fbc3896a5454927024a6bc87474026d5306971

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