[Object] Change Archive::findSym to return an Expected<Optional<Child>>.

As suggested by Rafael in review of D22079 - this was accidentally left out of
the final commit (r275316).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275469 91177308-0d34-0410-b5e6-96231b3b80d8