[clang] [clang][Sema] Fix crash on designated initializer of union member in subobject (PR #167171)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

Hi, thank you for this patch!
However I think there might be a bug/confustion somewhere else. Why would `getInitializedFieldInUnion` return non null when there is no initializer stored? Was it forgotten somewhere else? I would be more comfortable accepting this if we could explain why this happens.

https://github.com/llvm/llvm-project/pull/167171