DIE: Pass ownership of children via std::unique_ptr rather than raw pointer.

This should reduce the chance of memory leaks like those fixed in
r207240.

There's still some unclear ownership of DIEs happening in DwarfDebug.
Pushing unique_ptr and references through more APIs should help expose
the cases where ownership is a bit fuzzy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207263 91177308-0d34-0410-b5e6-96231b3b80d8