fix(IgxInputDirective): Calculate required value when input is bound via formControl

[teodosiah]

Closes #12895

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

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• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
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• This PR contains breaking changes
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• This PR includes behavioral changes and the feature specification has been updated with them

[jackofdiamond5]

Funny how the native validation passes when you blur the required textarea but fails for the required input. I managed to get the textarea to become invalid after deleting its value but imo it should become invalid on blur as well, the same way the input does, though that's not really in our control I guess. Reading the spec on textarea and input shows some differences but nothing explaining why this works the way it does. Oh well.