[llvm] [TableGen] Add `!records` operator to get defined records (PR #129680)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > I don't like the name `!defined`. Maybe `!instances` would be better? Anyone else have ideas?
> 
> Maybe `records`? `record` is the terminology used in https://llvm.org/docs/TableGen/ProgRef.html.

As an aside about naming: I know that record are called "records". But the name of this operator should suggest not just "records"; it should suggest "list of all records derived from a specified class".

https://github.com/llvm/llvm-project/pull/129680