Placeholder does not work for date fields

The date input field has been added in #2821483: Additional field types for D8, but it does not work because placeholders are not supported for date fields by the browsers/html5, you see always a date mask like dd/mm/jjjj.

Also the library of Mathias Bynens is not proposing placeholders for date fields: https://mathiasbynens.github.io/jquery-placeholder/

The HTML Standard says in 4.10.5.1.7 Date state (type=date)

when an input element's type attribute is in the Date state, the rules in this section apply. The following content attributes must not be specified and do not apply to the element: accept, ... , placeholder, ..., and width.

There are some hacky jquery workarounds, so if date should be allowed in this module one of these should then be implemented.