[ORC] Add a WrappedHandlerReturn type to map handler return types onto error

return types.

This class allows user provided handlers to return either error-wrapped types
or plain types. In the latter case, the plain type is wrapped with a success
value of Error or Expected<T> type to fit it into the rest of the serialization
machinery.

This patch allows us to remove the RPC unit-test workaround added in r286646.

llvm-svn: 286701