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

[Pengcheng Wang via llvm-commits]

wangpc-pp 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.

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