Handle aliasing a non-top-level command.

This didn't work previously because we had to check whether the incoming
command was an alias command, but if it wasn't we still used the result
of that lookup - which was by the command's node name.  That fails for
non-top-level commands.  In this case, the resolution is pretty simple since
we already have the node's CommandObject, so all we needed to do was turn
it into a shared pointer, for which I added enable_shared_from_this to the
CommandObject.

Differential Revision: https://reviews.llvm.org/D137662