UseListOrder: Try to resolve buildbot failure

MSVC [1] thinks `UseListShuffleVector` needs a copy constructor, but I
don't.  Let's see if being explicit about `UseListOrder` is convincing.

[1]: http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/11664/steps/build_Lld/logs/stdio

Here's the failure:

C:/lld-x86_64_win7/lld-x86_64-win7/llvm.src/include\llvm/IR/UseListOrder.h(92): error C2248: 'llvm::UseListShuffleVector::operator =' : cannot access private member declared in class 'llvm::UseListShuffleVector' (C:\lld-x86_64_win7\lld-x86_64-win7\llvm.src\lib\Bitcode\Writer\ValueEnumerator.cpp) [C:\lld-x86_64_win7\lld-x86_64-win7\llvm.obj\lib\Bitcode\Writer\LLVMBitWriter.vcxproj]
          C:/lld-x86_64_win7/lld-x86_64-win7/llvm.src/include\llvm/IR/UseListOrder.h(56) : see declaration of 'llvm::UseListShuffleVector::operator ='
          C:/lld-x86_64_win7/lld-x86_64-win7/llvm.src/include\llvm/IR/UseListOrder.h(32) : see declaration of 'llvm::UseListShuffleVector'
          This diagnostic occurred in the compiler generated function 'llvm::UseListOrder &llvm::UseListOrder::operator =(const llvm::UseListOrder &)'