Property initialisation checks fail when methods are in trait

### HHVM Version

hhvm: 3.28.3-0(brew)
hh_client: hh-d6bb63c6982aedf5c3c978f9097e066f5e208786-3.28.3

### Operating System and Version

MacOS High Sierra

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

```php
class A {
use T;

public function __construct() {
$this->setFoo();
}
}

trait T {
public string $foo;

private function setFoo() : void {
$this->foo = "fs";
}
}
```

### Actual result

```
test.hh:5:19,29: Class A does not initialize all of its members; foo is not always initialized.
Make sure you systematically set $this->foo when the method __construct is called.
Alternatively, you can define the member as optional (?...)
(NastCheck[3015])
test.hh:6:5,17: Until the initialization of $this is over, you can only call private methods
The initialization is not over because $this->foo can still potentially be null (NastCheck[3004])
```

### Expected result

No issues. If we inline `setFoo` into `A` explicitly the code passes:

```php
class A {
use T;

public function __construct() {
$this->setFoo();
}

private function setFoo() : void {
$this->foo = "fs";
}
}

trait T {
public string $foo;
}
```

I *think* the basic initialisation algorithm Hack's typechecker uses is: "a non-nullable property without a default is only validly initialised if defined in the `__construct`, or in a private method called in all branches of the `__construct`". It seems like private trait methods should be allowed as part of that.

This also isn't important - I just found it while playing around, and didn't see the behaviour documented.