Final private method in trait overridden by a child class of the using class is not caught by hh_client

After doing some work on this, I kinda think we should ban it on traits too. If we only ban final private methods on classes, hh will still accept this code that HHVM rejects:

trait MyTrait {
  public function foo(): void { $this->bar(); }
  final private function bar(): void {}
}

class MyClass {
  use MyTrait;
}

class MyChild extends MyClass {
  private function bar(): void {}
}

@lexidor do you have any opinions here?

Originally posted by @Wilfred in https://github.com/facebook/hhvm/issues/8805#issuecomment-845579117

The issue names no source files or tests. Start by reproducing the trait example with hh_client and HHVM, then trace the existing check for final private methods on classes. Done means the agreed trait behavior is documented by a regression test and hh_client no longer accepts code that HHVM rejects.