[PBQP] Re-commit r220642 with a workaround for quirky Visual Studio behavior.

Apparently unique_ptr'ifying NodeMetadata exposed an issue in VS where it
occasionally tries to synthesize copy constructors instead of moves. Hopefully
explicitly deleting the copy constructor and defining the move constructor will
fix this.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220649 91177308-0d34-0410-b5e6-96231b3b80d8