Use std::unique_ptr for DIE children

Got bored, removed some manual memory management.

Pushed references (rather than pointers) through a few APIs rather than
replacing *x with x.get().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206222 91177308-0d34-0410-b5e6-96231b3b80d8