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/CMakeLists.txt b/CMakeLists.txt
index 2d7bb6f..28f4e3f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -156,11 +156,14 @@
   endif()
 
   set( CLANG_BUILT_STANDALONE 1 )
+  set(BACKEND_PACKAGE_STRING "LLVM ${LLVM_PACKAGE_VERSION}")
+else()
+  set(BACKEND_PACKAGE_STRING "${PACKAGE_STRING}")
+endif()
 
-  find_package(LibXml2)
-  if (LIBXML2_FOUND)
-    set(CLANG_HAVE_LIBXML 1)
-  endif ()
+find_package(LibXml2)
+if (LIBXML2_FOUND)
+  set(CLANG_HAVE_LIBXML 1)
 endif()
 
 set(CLANG_RESOURCE_DIR "" CACHE STRING