bug(Form Field): Legacy form field input missing placeholder attribute

[JoeMellon]

Reproduction

Legacy form-field appearance from Material examples: https://stackblitz.com/angular/bqmpxokjgve?file=src%2Fapp%2Fform-field-appearance-example.html

Steps to reproduce:

1. View example above
2. Inspect input element from legacy form field example

Expected Behavior

placeholder attribute to be available in the dom

Actual Behavior

placeholder attribute missing from the dom

Environment

• Angular: 10.2.0
• CDK/Material: 10.2.6
• Browser(s): Chrome / Edge
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

[crisbeto]

For context, the form field with the legacy appearance has a feature where it turns the placeholder into the form field label, if the form field doesn't have have a mat-label element. This is a problem for screen readers, because they'll read out the same label twice on focus. In order to resolve the accessibility issue, a couple of months ago we started removing the placeholder from the DOM completely.

That being said, we might've been a bit too aggressive. Right now we're using the _hideControlPlaceholder method which hides the placeholder when the label isn't floating. We should make it so the logic only runs for the legacy appearance and if there isn't a mat-label.