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

llvm-svn: 207593

llvm/lib/Bitcode/Writer/BitWriter.cpp

@@ -10,6 +10,7 @@
 #include "llvm-c/BitWriter.h"
 #include "llvm/Bitcode/ReaderWriter.h"
 #include "llvm/IR/Module.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;