Fix #5094: Add regression test

Author: nicolasstucki

tests/run/i5094.scala

@@ -0,0 +1,17 @@
+trait IO {
+  def c(x: Int): Int = ???
+}
+trait SO extends IO {
+  override final def c(x: Int): Int = ???
+}
+trait SOIO extends IO {
+  override def c(x: Int): Int = ???
+}
+trait SOSO extends SOIO with SO
+abstract class AS extends SO
+class L extends AS with SOSO
+object Test {
+  def main(args: Array[String]): Unit = {
+    new L
+  }
+}