🍒 [Support/VirtualFileSystem] Bring in CASReference.h because Swift/C++ interop complains about ObjectRef being undefined. (#9174)

Cherry-pick of f93fb9e that fixes https://github.com/swiftlang/swift-llvm-bindings on the `rebranch` scheme.

# Conflicts:
#	llvm/include/llvm/Support/VirtualFileSystem.h

Co-authored-by: Argyrios Kyrtzidis <[email protected]>

Author: MaxDesiatov

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -18,6 +18,9 @@
 #include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.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/Errc.h"
 #include "llvm/Support/Error.h"