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

llvm-svn: 207593

llvm/lib/Object/ELFYAML.cpp

@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Object/ELFYAML.h"
+#include "llvm/Support/Casting.h"
 
 namespace llvm {