Introduce some types and functions to make it easier to work with

the tblgen AST node hierarchies.

Not totally NFC because both of the emitters now emit in a different
order.  The type-nodes emitter now visits nodes in hierarchy order,
which means we could use range checks in classof if we had any types
that would benefit from that; currently we do not.  The AST-nodes
emitter now uses a multimap keyed by the name of the record; previously
it was using `Record*`, which of couse isn't stable across processes
and may have led to non-reproducible builds in some circumstances.