Forbid truthiness tests on Traversable interface types

Forbid truthiness tests on Traversable interface types

Summary:
D8786329 bans truthiness tests on all class types (particularly, concrete or abstract classes, but not interfaces) which implement Traversable but not Container, since any instance of such a class will always be truthy, even when empty. It does not emit an error for any interface type, but testing the truthiness of an interface type which implements `Traversable` but not `Container` is also a potential logic error.

For example, some functions take a Traversable and then test its truthiness in order to determine whether it is empty:

```
function f(Traversable<int> $x): void {
  if (!$x) {
    print('x was empty');
  } else {
    print('x was non-empty');
  }
}
```

But this is not correct--types which implement `Container` will be falsy when empty, but any other type won't be. The behavior when the sequence is empty changes based on the runtime type of `$x`.

There is no way to check whether a `Traversable` is empty in general without causing side effects (since generators implement `Traversable`).

It seems to be the case that many situations where we take a `Traversable` interface type and then check its truthiness are ones where we could simply use the `Container` or `KeyedContainer` interface instead.

When the current behavior really is what a user wants, they should write this instead:

```
function f(Traversable<int> $x): void {
  if ($x instanceof Container && C\is_empty($x)) {
    print('x was empty');
  } else {
    print('x was either a non-empty Container or some other Traversable');
  }
}
```

This change forbids truthiness tests on any interface type which implements `Traversable` but not `Container`.

Reviewed By: DavidSnider

Differential Revision: D8900763

fbshipit-source-id: b97af95694b843dc80e96513afbbb518738d001e