Update slang for LLVM 3.5 rebase (r209713).

Change-Id: If0f510085358d2e48c865257851d9ff7f5846eb8
diff --git a/llvm-rs-as.cpp b/llvm-rs-as.cpp
index 0367e23..9e61395 100644
--- a/llvm-rs-as.cpp
+++ b/llvm-rs-as.cpp
@@ -22,6 +22,7 @@
 #include "llvm/Bitcode/ReaderWriter.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Signals.h"