Update LLVM for rebase to r212749.

Includes a cherry-pick of:
r212948 - fixes a small issue with atomic calls

Change-Id: Ib97bd980b59f18142a69506400911a6009d9df18
diff --git a/unittests/ADT/ImmutableMapTest.cpp b/unittests/ADT/ImmutableMapTest.cpp
index 774581c..6a99884 100644
--- a/unittests/ADT/ImmutableMapTest.cpp
+++ b/unittests/ADT/ImmutableMapTest.cpp
@@ -36,8 +36,8 @@
   EXPECT_TRUE(S.isEmpty());
   EXPECT_FALSE(S2.isEmpty());
 
-  EXPECT_EQ(0, S.lookup(3));
-  EXPECT_EQ(0, S.lookup(9));
+  EXPECT_EQ(nullptr, S.lookup(3));
+  EXPECT_EQ(nullptr, S.lookup(9));
 
   EXPECT_EQ(10, *S2.lookup(3));
   EXPECT_EQ(11, *S2.lookup(4));