Sure up ownership passing of the PBQPBuilder by passing unique_ptrs by value rather than lvalue reference.

Also removes an unnecessary '.release()' that should've been a std::move
anyway. (I'm on a hunt for '.release()' calls)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213464 91177308-0d34-0410-b5e6-96231b3b80d8