DwarfUnit: return by reference from createAndAddDIE

Since this doesn't return ownership (the DIE has been added to the
specified parent already) nor return null, just return by reference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207259 91177308-0d34-0410-b5e6-96231b3b80d8