Improve -Wunused:locals,privates by tracking use of `var`s (if set or not)

[PaulCoral]

In merged PR #16157, -Wunused:locals -Wunused:privates only checks if the definitions are used (read).

As suggested in the mentioned PR, the idea would be to improve these warnings as in Scala 2 -Ywarn-unused:<args> and to track also whether vars are set or not, see this Scala 2 test example.