[BUGFIX release] Fix setDiff computed macro used within glimmer component

[cloutierlp]

Using setDiff computed macro within a glimmer component is failing due to this.get being unaccessible within that scope.

Replacing the this.get with the get imported from @ember/-internals/metal just as in every other macros fixes the issue.

As every reduce_computed_macros tests are within the object folder, I didn't really know where to create the new failing test for the current issue, as it seems related to the usage of a glimmer component. If anyone have any idea on how and where we should create a test for this, I'd be interested to know.

To reproduce the issue, create a glimmer component and use the setDiff computed macro:

import Component from '@glimmer/component';
import { setDiff } from '@ember/object/computed';

export default class SetDiffComponent extends Component {
  fruits = [
    'banana',
    'grape',
    'kale'
  ]

  likes = [
    'grape',
    'kale'
  ]

  @setDiff('fruits', 'likes') wants;
}

And then trigger the computation of the wants property. This will be resulting in the following error:

Uncaught (in promise) TypeError: this.get is not a function
    at SetDiffComponent.<anonymous> (reduce_computed_macros.js:1114)
    at index.js:2478
    at untrack (index.js:1018)
    at ComputedProperty.get (index.js:2477)
    at SetDiffComponent.CPGETTER_FUNCTION [as wants] (index.js:638)
    at getPossibleMandatoryProxyValue (index.js:1722)
    at get (index.js:1788)
    at index.js:460
    at runInAutotrackingTransaction (index.js:706)
    at track (index.js:986)

[pzuraq]

Thanks for submitting this!