Update Clang for rebase to r212749.

This also fixes a small issue with arm_neon.h not being generated always.

Includes a cherry-pick of:
r213450 - fixes mac-specific header issue
r213126 - removes a default -Bsymbolic on Android

Change-Id: I2a790a0f5d3b2aab11de596fc3a74e7cbc99081d
diff --git a/unittests/Tooling/CommentHandlerTest.cpp b/unittests/Tooling/CommentHandlerTest.cpp
index f0f7797..117dfc3 100644
--- a/unittests/Tooling/CommentHandlerTest.cpp
+++ b/unittests/Tooling/CommentHandlerTest.cpp
@@ -28,7 +28,7 @@
   typedef TestVisitor<CommentHandlerVisitor> base;
 
 public:
-  CommentHandlerVisitor() : base(), PP(0), Verified(false) { }
+  CommentHandlerVisitor() : base(), PP(nullptr), Verified(false) {}
 
   ~CommentHandlerVisitor() {
     EXPECT_TRUE(Verified) << "CommentVerifier not accessed";