Update llvm-auto-clean-trigger

Update the datestamp so that buildbots do a clean rebuild of llvm.

Author: raphlinus

src/rustllvm/llvm-auto-clean-trigger

@@ -1,4 +1,4 @@
 # If this file is modified, then llvm will be forcibly cleaned and then rebuilt.
 # The actual contents of this file do not matter, but to trigger a change on the
 # build bots then the contents should be changed so git updates the mtime.
-2016-10-10b
+2016-10-18