[Object] Include llvm/Support/MemoryBuffer.h after D119457

libc++ -DLLVM_ENABLE_MODULES=on build needs the complete type. This fixes

error: invalid application of 'sizeof' to an incomplete type 'llvm::MemoryBuffer'

llvm/include/llvm/Object/Binary.h

@@ -18,14 +18,13 @@
 #include "llvm/Object/Error.h"
 #include "llvm/Support/CBindingWrapping.h"
 #include "llvm/Support/Error.h"
-#include "llvm/Support/MemoryBufferRef.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include <memory>
 #include <utility>
 
 namespace llvm {
 
 class LLVMContext;
-class MemoryBuffer;
 class StringRef;
 
 namespace object {