[Support/VirtualFileSystem] Bring in CASReference.h because Swift/C++ interop complains about ObjectRef being undefined.

Author: akyrtzi

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -20,6 +20,9 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/STLFunctionalExtras.h"
+// FIXME: Bringing in "CASReference.h" because Swift/C++ interop complains about
+// `ObjectRef` being undefined.
+#include "llvm/CAS/CASReference.h"
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorOr.h"