Enable diamond import of final methods via `use` chains

Enable diamond import of final methods via `use` chains

Summary:
This diff enables diamond inclusion of traits via `use` chains should be supported even if they define final methods.  Eg. the code below should be accepted:

```
trait T {
    final public function foo() : int { return 1; }
}

trait T1 { use T; }
trait T2 { use T; }

<<__EnableMethodTraitDiamond>>
class C {
    use T1, T2;
}
```

The rationale is that `final` is a constraint about inheritance hierarchy.  We thus still reject:

```
trait T {
    final public function foo() : int { return 1; }
}

class D  { use T; }

<<__EnableMethodTraitDiamond>>
class C extends D {
    use T;
}
```
because this code reuses a final method via the class inheritance.

Reviewed By: Wilfred

Differential Revision: D31857847

fbshipit-source-id: 0661d257e513a27bd5ac7070b8d6cd498d00f489