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

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.