Backport "Fix compiler crash in WUnused" (#17356)

Backports #17340

Author: Kordyjan

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -46,6 +46,7 @@ class CheckUnused private (phaseMode: CheckUnused.PhaseMode, suffix: String, _ke
   override def description: String = CheckUnused.description
 
   override def isRunnable(using Context): Boolean =
+    super.isRunnable &&
     ctx.settings.Wunused.value.nonEmpty &&
     !ctx.isJava
 

tests/neg-custom-args/fatal-warnings/i17335.scala

@@ -0,0 +1,4 @@
+// scalac: -Wunused:all
+
+def aMethod() =
+  doStuff { (x) => x } // error