[ORC][RPC] Make the future type of an Orc RPC call Error/Expected rather than

Optional.

For void functions the return type of a nonblocking call changes from
Expected<future<Optional<bool>>> to Expected<future<Error>>, and for functions
returning T the return type changes from Expected<future<Optional<T>>> to
Expected<future<Expected<T>>>.

Inner results need to be checked (since the RPC connection may have dropped
out before a result came back) and Error/Expected provide stronger checking
requirements. It also allows us drop the crufty 'optionalToError' function and
just collapse Errors in the single-threaded call primitives.

llvm-svn: 280016