left-associative ternary operator

Hack seems to follow PHP in this unusual behavior, but since PHP 5.x support is gone, its time to fix this.

Similar to what @[nikic] proposed for PHP 7.4, ~the type checker~ *HHVM* can force using parentheses with nested ternary operations, and a change can be made in the future to make the ternary operation right-associative.

links :

- https://bugs.php.net/bug.php?id=61915
- https://externals.io/message/105163
- https://wiki.php.net/rfc/ternary_associativity

---

### HHVM Version
```console
➜ framework git:(http) ✗ hhvm --version
HipHop VM 4.1.0 (rel)
Compiler: 1555455032_861766839
Repo schema: b1f582ab39dca7cf649292454f7b664970bf83d9
➜ framework git:(http) ✗ hh_client --version
hh-21577913b41b950cbe2cd887f46cb30964e59cbd-4.1.0 Tue Apr 16 23:55:04 2019
➜ framework git:(http) ✗

```
### Operating System and Version

irrelevant

### Standalone code, or other way to reproduce the problem

```
<<__EntryPoint>>
function main(): void {
$order = "3";
$food = ($order == '1') ? 'Banana' :
($order == '2') ? 'Apple' :
($order == '3') ? 'Toast' :
($order == '4') ? 'Cantalope' :
($order == '5') ? 'Swiss Cheese' : 'Fig Newton Cookie';

print $food . "\n";
}

```
### Actual result

```
Swiss Cheese
```

### Expected result
```
Toast
```