Revert "[PBQP] Unique-ptrify some PBQP Metadata structures. No functional change." (r220642)

It broke the Windows build:

  [1/19] Building CXX object lib\CodeGen\CMakeFiles\LLVMCodeGen.dir\RegAllocPBQP.cpp.obj
  C:\bb-win7\ninja-clang-i686-msc17-R\llvm-project\llvm\include\llvm/CodeGen/RegAllocPBQP.h(132) : error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

     with
     [
         _Ty=unsigned int []
     ]
     D:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\memory(1600) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr'
     with
     [
         _Ty=unsigned int []
     ]
     This diagnostic occurred in the compiler generated function 'llvm::PBQP::RegAlloc::NodeMetadata::NodeMetadata(const llvm::PBQP::RegAlloc::NodeMetadata &)'