[clang] [CIR] Upstream minimal support for structure types (PR #135105)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

@erichkeane Are you happy with this in its current state? I will make changes to change the polarity of complete/incomplete and to merge RecordKind::class and RecordKind::struct in the incubator and then merge them upstream if no issues are found.

https://github.com/llvm/llvm-project/pull/135105