SI-7624 Fix -feature warnings and build with -feature

I left one warning untouched, because instead of adding the import the
diagnostics should be fixed:

    scala/src/library/scala/collection/parallel/mutable/LazyCombiner.scala:33:
    warning: the existential type
        scala.collection.parallel.mutable.LazyCombiner[_$1,_$2,_$3] forSome {
          type _$1; type _$2; type _$3 <: scala.collection.generic.Growable[_$1] with scala.collection.generic.Sizing },
    which cannot be expressed by wildcards, should be enabled by making the implicit value scala.language.existentials visible.
    if (other.isInstanceOf[LazyCombiner[_, _, _]]) {
                       ^