Floating labels in input groups

[hanneswylaers]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Regular inputs can be wrapped in a input-group. When doing this, some small css tweaks are being done e.g. setting the correct rounding on the input, increasing the z-index of the focus on the input, ...

The suggestion is to also allow floating labels to be used in an input-group.
Right now, when using a form floating input in combination with a button in an input group,

• the right-upper and right-lower corner is still rounded (when button is at end),
• the focus of the input is appearing below the button, while on regular input, the z-index is increased to 3.
  

Motivation and context

We are using a floating input password field in combination with a button in an input group to show/hide the password.
It's a (simple) addition on what is currently existing.

[mdo]

Duplicate of #34513.