[clang] [CIR] Introduce type aliases for records (PR #136387)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

Seems reasonable to me, but not the best one to review this.  

I question stealing `ty` as the prefix to this instead of `rt` (or something more specific to records), but I'm not sure I feel particularly strongly.  Just it seems that `ty` is going to be pretty meaningless perhaps?

ALSO, I see no tests that contain `ty_anon`, can you write some?

Finally: Why the prefix at all?

https://github.com/llvm/llvm-project/pull/136387