DwarfUnit: Suddently, DIE references, everywhere.

This'll make changing to unique_ptr ownership of DIEs easier since the
usages will now have '*' on them making them textually compatible
between unique_ptr and raw pointer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207253 91177308-0d34-0410-b5e6-96231b3b80d8
SCC: Remove non-const operator*()

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207254 91177308-0d34-0410-b5e6-96231b3b80d8