Revert "unique_ptrify ValID::ConstantStructElts"

Revert "unique_ptrify ValID::ConstantStructElts"

This reverts r231200 and r231204. The second one added an explicit move
ctor for MSVC.

This change broke the clang-cl self-host due to weirdness in MSVC's
implementation of std::map::insert. Somehow we lost our rvalue ref-ness
when going through variadic placement new:

  template <class _Objty, class... _Types>
  void construct(_Objty *_Ptr,
                 _Types &&... _Args) { // construct _Objty(_Types...) at _Ptr
    ::new ((void *)_Ptr) _Objty(_STD forward<_Types>(_Args)...);
  }

For some reason, Clang decided to call the deleted std::pair copy
constructor at this point. Needs further investigation, once I can
build.

llvm-svn: 231269