Enable IntMap in scala-collections-whitelist

Author: odersky

test/dotc/scala-collections.whitelist

@@ -210,7 +210,7 @@
 #./scala-scala/src/library/scala/collection/immutable/HashSet.scala
 
 # https://github.com/lampepfl/dotty/issues/939
-#./scala-scala/src/library/scala/collection/immutable/IntMap.scala
+./scala-scala/src/library/scala/collection/immutable/IntMap.scala
 
 
 ./scala-scala/src/library/scala/collection/immutable/ListMap.scala