Add Optional.h to BlotSetVector

It was getting included transitively before, but it isn't anymore due to
refactorings in llvm.

Author: etcwilde

include/swift/Basic/BlotSetVector.h

@@ -15,6 +15,7 @@
 
 #include "swift/Basic/LLVM.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 #include <vector>