[ORC] Fix unchecked Expected<T> in ELFDebugObjectPlugin::FinalizePromise (#172904)
If `Alloc.finalize()` fails in the post-allocation pass, we store the
error in `FinalizePromise`. If we don't reach the post-fixup pass
afterwards the error will leak. This patch adds another case in the
DebugObject destructor that will check the `Expected<T>` and report the
error.