Fix #3557: Add regression test

The test now compiles with errors, which is what we would expect.

Author: odersky

tests/neg/i3557.scala

@@ -0,0 +1,14 @@
+
+class C1 {
+  @annotation.varargs
+  def f(values: String*) = ()
+}
+class C2 {
+  @scala.annotation.varargs
+  def f(values: String*) = ()
+  def g: String => Int = s => hashCode
+  class C3 {
+    @f.varargs                    // error
+    def f(values: String) = ()
+  }
+}