Fix AttrNoOverride* on classes

Fix AttrNoOverride* on classes

Summary:
We weren't checking for uniqueness before setting the magic method
no-override flags, which could in theory result in setting them
incorrectly.

eg if there's a single class B which extends A, with two definitions
of A, and a class C which defines a magic getter, and extends B, but
which is incompatible with one of the As, we'll incorrectly set the
no-override bit on B because *one* possible instantiation of B can't
have a magic getter.

I couldn't come up with code that behaves incorrectly, but

  <?hh
  class A {};
  class B extends A {};
  class C extends B {
    const FOO = 42;
    public function __get($n) { echo "getting $n\n"; return 42; }
  }
  function f(B $b) {
    return $b->foo;
  }
  <<__EntryPoint>>
  function main() {
    if (false) {
      include "bug.inc";
    }
    var_dump(f(new C));
  }

with bug.inc:

  <?hh
  class A {
    const type FOO = int;
  };

Gets the wrong flags on B without this diff.

Reviewed By: ricklavoie, alexeyt

Differential Revision: D15966816

fbshipit-source-id: 4b4a94e7a5107213377849d9e8ae5d9d1f32105e