[PBQP] Explicitly define copy/move operations for NodeMetadata to keep VS happy.

Hopefully this fixes the bug that was introduced in r220642, and not-quite-fixed
in r220649.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220653 91177308-0d34-0410-b5e6-96231b3b80d8