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/llvm-device-build.mk b/llvm-device-build.mk
index 82ef055..5d863f5 100644
--- a/llvm-device-build.mk
+++ b/llvm-device-build.mk
@@ -64,6 +64,8 @@
 	$(LLVM_ROOT_PATH)/device/include	\
 	$(LOCAL_C_INCLUDES)
 
+include external/libcxx/libcxx.mk
+
 ###########################################################
 ## Commands for running tblgen to compile a td file
 ###########################################################