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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

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

We don't actually generate anon records (either upstream or in the incubator). I think we should just make them illegal. Anonymous structs in the source are given a name in CIR (`anon.<n>`)

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