Add default constructors for iterators.

These iterators don't point anywhere, and they can't be compared to anything.
They are only good for assigning to.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120239 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/ADT/IntervalMapTest.cpp b/unittests/ADT/IntervalMapTest.cpp
index f4b1ebc..6b8e761 100644
--- a/unittests/ADT/IntervalMapTest.cpp
+++ b/unittests/ADT/IntervalMapTest.cpp
@@ -41,6 +41,14 @@
   UUMap::iterator I = map.begin();
   EXPECT_FALSE(I.valid());
   EXPECT_TRUE(I == map.end());
+
+  // Default constructor and cross-constness compares.
+  UUMap::const_iterator CI;
+  CI = map.begin();
+  EXPECT_TRUE(CI == I);
+  UUMap::iterator I2;
+  I2 = map.end();
+  EXPECT_TRUE(I2 == CI);
 }
 
 // Single entry map tests