[clang] [clang][analyzer] Fix the false positive ArgInitializedness warning on unnamed bit-field (PR #145066)

[Balazs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

Thank you for this PR. The PR summary explains the situation well and the patch also looks correct to me modulo inline comments.

Did you pick up some issue, or just decided to fix this because it was annoying to you?
If we have a related ticket, we should link it here and close it once we are done here.

https://github.com/llvm/llvm-project/pull/145066