Fix false positives about unreferenced class elements in subclasses for static:: (constants, methods, properties)

[TysonAndre]

Reported on gitter today

class A {
  public function foo() {
    return static::SOME_CONST;
  }
}

class B extends A {
  protected const SOME_CONST = 'hello, world';
}

Expected: PhanUndeclaredConstantOfClass is emitted, because (new A())->foo() (and so on) can be called.
Observed: PhanUnreferencedProtectedClassConstant and PhanUndeclaredConstantOfClass is emitted

This is causing PhanUnreferencedProtectedClassConstant for B::SOME_CONST. Shouldn't phan be able to see the usage in A? using 2.5.0 (and 2.6.1-dev)

Currently, Phan can't track this reference, because it only tracks references to elements that exist

Phan could track references to properties, methods, and class constants that don't exist separately from those that do exist, so that the reference to static::SOME_CONST, static::$some_prop, and static::someMethod() can be properly recorded.

Note that Phan already warns about the constant not existing in the base class