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

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> 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?

Good point, `rec` or `rt` probably make more sense. I'm fine if this change comes in the future in case it's a lot of noise for the changes required on existing tests here and in the incubator.

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