SI-7020 Determinism for pattern matcher warnings

Use LinkedHashSet for the DPLL algorithm for determistic
counter example generation.

Before, the test compiled with:

    [info] v2.10.2 => /Users/jason/usr/scala-v2.10.2-0-g60d462e
    test/files/neg/t7020.scala:3: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:10: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 5, 6, 7))), List(_, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:17: warning: match may not be exhaustive.
    It would fail on the following inputs: List((x: Int forSome x not in (1, 2, 4, 6, 7)), _), List(1, _), List(2, _), List(4, _), List(5, _), List(6, _), List(7, _), List(??, _)
      List(5) match {
          ^
    test/files/neg/t7020.scala:24: warning: match may not be exhaustive.
    It would fail on the following input: List(_, _)
      List(5) match {
          ^

1 files changed, 30 insertions, 0 deletions

diff --git a/test/files/neg/t7020.scala b/test/files/neg/t7020.scala
new file mode 100644
index 0000000000..cc5421bab1
--- /dev/null
+++ b/test/files/neg/t7020.scala
@@ -0,0 +1,30 @@
+object Test {
+  // warning was non-deterministic
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+
+  List(5) match {
+    case 1 :: Nil | 2 :: Nil  =>
+    case (x@(4 | 5 | 6)) :: Nil =>
+    case 7 :: Nil  =>
+    case Nil =>
+  }
+}