Update clang to r339409.

Change-Id: I800772d2d838223be1f6b40d490c4591b937fca2
diff --git a/linux-x64/clang/include/llvm/Support/ThreadPool.h b/linux-x64/clang/include/llvm/Support/ThreadPool.h
index fb82559..4fdbd52 100644
--- a/linux-x64/clang/include/llvm/Support/ThreadPool.h
+++ b/linux-x64/clang/include/llvm/Support/ThreadPool.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_SUPPORT_THREAD_POOL_H
 #define LLVM_SUPPORT_THREAD_POOL_H
 
+#include "llvm/Config/llvm-config.h"
 #include "llvm/Support/thread.h"
 
 #include <future>