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/docs/PTHInternals.rst b/docs/PTHInternals.rst
index 10dda61..7401cf9 100644
--- a/docs/PTHInternals.rst
+++ b/docs/PTHInternals.rst
@@ -135,11 +135,11 @@
 an algorithmic level, especially when one considers header files of
 arbitrary size.
 
-There are plans to potentially implement an complementary PCH
-implementation for Clang based on the lazy deserialization of ASTs. This
-approach would theoretically have the same constant-time algorithmic
-advantages just mentioned but would also retain some of the strengths of
-PTH such as reduced memory pressure (ideal for multi-core builds).
+There is also a PCH implementation for Clang based on the lazy
+deserialization of ASTs. This approach theoretically has the same
+constant-time algorithmic advantages just mentioned but also retains some
+of the strengths of PTH such as reduced memory pressure (ideal for
+multi-core builds).
 
 Internal PTH Optimizations
 --------------------------