raw_ostream: Forward declare OpenFlags and include FileSystem.h only where necessary.

llvm-svn: 207593

llvm/tools/llvm-dis/llvm-dis.cpp

@@ -25,6 +25,7 @@
 #include "llvm/IR/Type.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/DataStream.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/FormattedStream.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"