assert that a component tagName can not be a computed property

[GavinJoyce]

fixes #14749

Component.extend({
  tagName: computed(function() {
    return 'foo-bar';
  })
});

Now produces an assertion:

You cannot use a computed property for the component's `tagName` (<(subclass of Ember.Component):ember445>)

[locks]

@GavinJoyce do you mean "for the component's tagName"? Otherwise it sounds like the component is named tagName to me.

[GavinJoyce]

@locks that's better, thanks

[rwjblue]

Thanks @GavinJoyce!