Fix failing main due to wunused (#17263)

Merging the last pull request made the main red. There was an undetected
conflict between two PRs.

Author: szymon-rd

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

@@ -118,7 +118,7 @@ class CheckUnused private (phaseMode: CheckUnused.PhaseMode, suffix: String, _ke
 
   override def prepareForDefDef(tree: tpd.DefDef)(using Context): Context =
     unusedDataApply{ ud =>
-      if !tree.rawMods.is(Private) then
+      if !tree.symbol.is(Private) then
         tree.termParamss.flatten.foreach { p =>
           ud.addIgnoredParam(p.symbol)
         }

tests/neg-custom-args/fatal-warnings/i15503i.scala

@@ -46,9 +46,6 @@ package foo.test.scala.annotation:
   def a1(a: Int) = a // OK
   def a2(a: Int) = default_int // ok
 
-  private def a2_p(a: Int) = default_int // error
-  def a2_p_used = a2_p(3)
-
   def a3(@unused a: Int) = default_int //OK
 
   def b1 =