[llvm-remarkutil] Fix building with Xcode 12

This fixes erorrs like these:

llvm-project/llvm/tools/llvm-remarkutil/RemarkCounter.h:90:14: error: call to deleted constructor of 'llvm::Error'
      return E;
             ^
llvm-project/llvm/include/llvm/Support/Error.h:189:3: note: 'Error' has been explicitly marked deleted here
  Error(const Error &Other) = delete;
  ^
llvm-project/llvm/include/llvm/Support/Error.h:496:18: note: passing argument to parameter 'Err' here
  Expected(Error Err)
                 ^