Update clang to r339409b.

Change-Id: Ied8a188bb072c40035320acedc86164b66d920af
diff --git a/linux-x64/clang/include/llvm-c/lto.h b/linux-x64/clang/include/llvm-c/lto.h
index 1acd610..090cd34 100644
--- a/linux-x64/clang/include/llvm-c/lto.h
+++ b/linux-x64/clang/include/llvm-c/lto.h
@@ -44,7 +44,7 @@
  * @{
  */
 
-#define LTO_API_VERSION 22
+#define LTO_API_VERSION 23
 
 /**
  * \since prior to LTO_API_VERSION=3
@@ -828,6 +828,16 @@
                                                  unsigned max_size_bytes);
 
 /**
+ * Same as thinlto_codegen_set_cache_size_bytes, except the maximum size is in
+ * megabytes (2^20 bytes).
+ *
+ * \since LTO_API_VERSION=23
+ */
+extern void
+thinlto_codegen_set_cache_size_megabytes(thinlto_code_gen_t cg,
+                                         unsigned max_size_megabytes);
+
+/**
  * Sets the maximum number of files in the cache directory. An unspecified
  * default value will be applied. A value of 0 will be ignored.
  *