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/tools/libclang/CXCompilationDatabase.cpp b/tools/libclang/CXCompilationDatabase.cpp
index f4728fe..51677e7 100644
--- a/tools/libclang/CXCompilationDatabase.cpp
+++ b/tools/libclang/CXCompilationDatabase.cpp
@@ -54,7 +54,7 @@
       return new AllocatedCXCompileCommands(std::move(CCmd));
   }
 
-  return 0;
+  return nullptr;
 }
 
 CXCompileCommands
@@ -65,7 +65,7 @@
       return new AllocatedCXCompileCommands(std::move(CCmd));
   }
 
-  return 0;
+  return nullptr;
 }
 
 void
@@ -90,13 +90,13 @@
 clang_CompileCommands_getCommand(CXCompileCommands Cmds, unsigned I)
 {
   if (!Cmds)
-    return 0;
+    return nullptr;
 
   AllocatedCXCompileCommands *ACC =
     static_cast<AllocatedCXCompileCommands *>(Cmds);
 
   if (I >= ACC->CCmd.size())
-    return 0;
+    return nullptr;
 
   return &ACC->CCmd[I];
 }