Fix #14667: Remove mangling of inner AnyQualifiedName's in fullNameSeparated.

This brings the `case name @ AnyQualfiedName(_, _)` in line with
the behavior of `case naem: SimpleName`, which already did not
mangle before this change.

This fixes #14667 because `Mixin.traitSetterName` indirectly calls
`fullNameSeparated`, and expanded private symbolic member names
would get mangled, and therefore could not identify their
corresponding fields anymore.

Author: gagandeepkalra

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -488,7 +488,7 @@ object SymDenotations {
           if kind == FlatName && !encl.is(JavaDefined) then qn.compactified else qn
         val fn = name replace {
           case name: SimpleName => qualify(name)
-          case name @ AnyQualifiedName(_, _) => qualify(name.mangled.toSimpleName)
+          case name @ AnyQualifiedName(_, _) => qualify(name.toSimpleName)
         }
         if (name.isTypeName) fn.toTypeName else fn.toTermName
       }

tests/run/i14667.check

@@ -0,0 +1,2 @@
+sigma
+alpha beta gamma

tests/run/i14667.scala

@@ -0,0 +1,16 @@
+trait A {
+  private val ? = "sigma"
+  def a = ?
+
+  val ?? = "alpha beta gamma"
+  def b = ??
+}
+
+class B extends A
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(new B().a)
+    println(new B().b)
+  }
+}