Convert the Archive API to use ErrorOr.

Now that we have c++11, even things like ErrorOr<std::unique_ptr<...>> are
easy to use.

No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211033 91177308-0d34-0410-b5e6-96231b3b80d8