Address binary incompatibilities

  - Restore default implementation of ClassTag.apply in 2.12.x
  - Whitelist the change to add a concrete implementation of toList
    in IndexedSeqOptimized.

I tested the latter change for potential breakage with:

```
object Test {
  def main(args: Array[String]) {
    def s = new collection.immutable.WrappedString("")
    s.toList
    (s: collection.IndexedSeqOptimized[Char, Any]).toList
    (s: collection.GenTraversableOnce[Char]).toList
  }
}

```

```
$ qscalac sandbox/test.scala && scala-launch 2.12.0 Test
$ scalac-launch 2.12.0 sandbox/test.scala && qscala Test
```

Author: retronym

library/src/mima-filters/2.12.0.forwards.excludes

@@ -45,3 +45,4 @@ ProblemFilters.exclude[MissingFieldProblem]("scala.collection.immutable.Map#Map1
 ProblemFilters.exclude[MissingFieldProblem]("scala.collection.immutable.Map#Map2.serialVersionUID")
 ProblemFilters.exclude[MissingFieldProblem]("scala.collection.immutable.Map#Map3.serialVersionUID")
 ProblemFilters.exclude[MissingFieldProblem]("scala.collection.immutable.Map#Map4.serialVersionUID")
+ProblemFilters.exclude[DirectAbstractMethodProblem]("scala.collection.GenTraversableOnce.toList")

library/src/scala/reflect/ClassTag.scala

@@ -46,7 +46,20 @@ trait ClassTag[T] extends ClassManifestDeprecatedApis[T] with Equals with Serial
   def wrap: ClassTag[Array[T]] = ClassTag[Array[T]](arrayClass(runtimeClass))
 
   /** Produces a new array with element type `T` and length `len` */
-  override def newArray(len: Int): Array[T]
+  override def newArray(len: Int): Array[T] = {
+    runtimeClass match {
+      case java.lang.Byte.TYPE      => new Array[Byte](len).asInstanceOf[Array[T]]
+      case java.lang.Short.TYPE     => new Array[Short](len).asInstanceOf[Array[T]]
+      case java.lang.Character.TYPE => new Array[Char](len).asInstanceOf[Array[T]]
+      case java.lang.Integer.TYPE   => new Array[Int](len).asInstanceOf[Array[T]]
+      case java.lang.Long.TYPE      => new Array[Long](len).asInstanceOf[Array[T]]
+      case java.lang.Float.TYPE     => new Array[Float](len).asInstanceOf[Array[T]]
+      case java.lang.Double.TYPE    => new Array[Double](len).asInstanceOf[Array[T]]
+      case java.lang.Boolean.TYPE   => new Array[Boolean](len).asInstanceOf[Array[T]]
+      case java.lang.Void.TYPE      => new Array[Unit](len).asInstanceOf[Array[T]]
+      case _                        => java.lang.reflect.Array.newInstance(runtimeClass, len).asInstanceOf[Array[T]]
+    }
+  }
 
   /** A ClassTag[T] can serve as an extractor that matches only objects of type T.
    *