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/PointerUnionTest.cpp b/unittests/ADT/PointerUnionTest.cpp
index 3bfb79c..a592784 100644
--- a/unittests/ADT/PointerUnionTest.cpp
+++ b/unittests/ADT/PointerUnionTest.cpp
@@ -66,7 +66,7 @@
 TEST_F(PointerUnionTest, Get) {
   EXPECT_EQ(a.get<float *>(), &f);
   EXPECT_EQ(b.get<int *>(), &i);
-  EXPECT_EQ(n.get<int *>(), (int *)0);
+  EXPECT_EQ(n.get<int *>(), (int *)nullptr);
 }
 
 } // end anonymous namespace