Fix #3675: don't unnecessarily wrap arrays when eliminating java varargs

When eliminating varargs in a java method call, we need to convert Seqs
into Arrays. However, if the object we're passing is already a wrapped
array, we just need to unwrap it.

Testing
-------

Eliminating varargs in

```
import java.nio.file._
class Test {
  def test(xs: Array[String]) = {
     val p4 = Paths.get("Hello", xs: _*)
  }
}
```

now gives

`val p4: java.nio.file.Path = java.nio.file.Paths.get("Hello", xs)`

Author: Abel Nieto

compiler/src/dotty/tools/dotc/transform/ElimRepeated.scala

@@ -94,6 +94,8 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
   private def seqToArray(tree: Tree, pt: Type)(implicit ctx: Context): Tree = tree match {
     case SeqLiteral(elems, elemtpt) =>
       JavaSeqLiteral(elems, elemtpt)
+    case app@Apply(fun, args) if isWrappedArray(app) =>
+      args.head // There's only one argument: the wrapped array
     case _ =>
       val elemType = tree.tpe.elemType
       var elemClass = elemType.classSymbol
@@ -106,6 +108,14 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
           // Because of phantomclasses, the Java array's type might not conform to the return type
   }
 
+  /** Determines whether the given `tree` represents a wrapped array */
+  private def isWrappedArray(tree: Apply)(implicit ctx: Context): Boolean = {
+    val elemTpe = tree.tpe.elemType
+    val wrapMethodName = TreeGen.wrapArrayMethodName(elemTpe)
+    val wrapMethodSym = defn.ScalaPredefModuleRef.denot.requiredMethod(wrapMethodName)
+    tree.fun.symbol == wrapMethodSym
+  }
+
   override def transformTypeApply(tree: TypeApply)(implicit ctx: Context): Tree =
     transformTypeOfTree(tree)