Add the linter option -Wunused:patvars

[PaulCoral]

In the (currently approved) PR #16157 (related to issue #15503), which add some linter options, it was decided not to add -Wunused:patvars yet, as it requires modifications to more phases of the compiler. Thus, it seems a better idea to handle it separately.

The current replacement is -Wunused:unsafe-warn-patvars which doesn't support suppressing warnings (x @ _), thus triggering false positive, i.e. in for expression.

The issue is that it seems not possible to differentiate these cases, after desugaring :

case Foo(x)   => ??? // desugared to Foo(x@_)
case Foo(x@_) => ???

TODO :

• Add -Wunused:patvars (which should also be triggered by -Wunused:all)
• remove -Wunused:unsafe-warn-patvars
• Update the test suits i15503*.scala
• Update the help message.

[Linyxus]

After discussing this issue on today's LAMP compiler meeting, we decide to close it as not planned.