unique_ptrify RuntimeDyldImpl::loadObject

I'm not sure this is a particularly helpful API (to pass ownership and
then return it unconditionally) rather than just pass the underlying
object by non-const reference, but this was the original API so I'll
just make it more safe/stable and anyone else is free to adjust that at
their whim, of course.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217081 91177308-0d34-0410-b5e6-96231b3b80d8