This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.
I'm planning to deprecate and eventually remove
StringRef::{starts,ends}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.
