Improve explicit memory ownership of DIEs

Now that the subtle constructScopeDIE has been refactored into two
functions - one returning memory to take ownership of, one returning a
pointer to already owning memory - push unique_ptr through more APIs.

I think this completes most of the unique_ptr ownership of DIEs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207442 91177308-0d34-0410-b5e6-96231b3b80d8