Update LLVM to fix nightly build failures

src/llvm

@@ -1 +1 @@
-Subproject commit 168f91ce5cbf8933e47f339911f0f46a48714852
+Subproject commit 3c88bf78f97d2cd8130b16eecb0eeb84888cd494

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.
-2015-09-02
+2015-09-11

src/test/run-pass/allocator-override.rs

@@ -11,12 +11,16 @@
 // no-prefer-dynamic
 // aux-build:allocator-dummy.rs
 
+#![feature(test)]
+
 extern crate allocator_dummy;
+extern crate test;
 
 fn main() {
     unsafe {
         let before = allocator_dummy::HITS;
-        let b = Box::new(3);
+        let mut b = Box::new(3);
+        test::black_box(&mut b); // Make sure the allocation is not optimized away
         assert_eq!(allocator_dummy::HITS - before, 1);
         drop(b);
         assert_eq!(allocator_dummy::HITS - before, 2);