Roll back clang revision.

This picked up the Clang regression which broke the MSan bots. See
https://github.com/google/sanitizers/issues/993.

Change-Id: I9882efe3b7a2f62795e4387038008256744d6f57
Reviewed-on: https://boringssl-review.googlesource.com/31945
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/util/bot/update_clang.py b/util/bot/update_clang.py
index 3a12b25..0a91189 100644
--- a/util/bot/update_clang.py
+++ b/util/bot/update_clang.py
@@ -19,7 +19,7 @@
 # CLANG_REVISION and CLANG_SUB_REVISION determine the build of clang
 # to use. These should be synced with tools/clang/scripts/update.py in
 # Chromium.
-CLANG_REVISION = '340925'
+CLANG_REVISION = '337439'
 CLANG_SUB_REVISION=1
 
 PACKAGE_VERSION = "%s-%s" % (CLANG_REVISION, CLANG_SUB_REVISION)