Override getOrElseUpdate in concurrent.Map

[scabug]

The concurrent.Map trait should override getOrElseUpdate as follows:

override def getOrElseUpdate(key: A, op: => B): B = get(key) match {
  case Some(v) => v
  case None =>
    val v = op
    putIfAbsent(key, v).getOrElse(v)
}

This is a continuation of #7943, where the more complete discussion lives.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9147?orig=1
Reporter: @axel22
Affected Versions: 2.11.4

[scabug]

@adriaanm said:
scala/scala#4323