Move include to the proper place. No functionality change

llvm-svn: 111204
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 41c5dbc..c66f08d 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -32,6 +32,7 @@
 #include "clang/Basic/TargetOptions.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Basic/Diagnostic.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/System/Host.h"
 #include "llvm/System/Path.h"