Test the interaction of mixin forwarders and overloads

Not only does this test still pass now that mixin forwarders are emitted
as bridge: it used to cause an ambiguous overload error on ecj and now
works correctly.

Author: smarter

test/files/run/mixin-forwarder-overload.check

@@ -0,0 +1 @@
+Foo(0)

test/files/run/mixin-forwarder-overload/A.scala

@@ -0,0 +1,9 @@
+case class Foo(x: Int)
+
+trait A[X]  {
+  def concat[Dummy](suffix: Int): Dummy = ???
+}
+
+class Bar extends A[Foo] {
+  def concat(suffix: Int): Foo = Foo(0)
+}

test/files/run/mixin-forwarder-overload/Test.java

@@ -0,0 +1,7 @@
+public class Test {
+  public static void main(String[] args) {
+    Bar bar = new Bar();
+    Foo x = bar.concat(0);
+    System.out.println(x);
+  }
+}