[llvm] [clang-tools-extra] [clang] [clang] Fix false positive -Wmissing-field-initializer for anonymous unions (PR #70829)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think the changes here look reasonable. CC @zygoloid to make sure he agrees as there's still one potentially outstanding comment (if we don't hear back from Richard, I think we can address his concerns post-commit as it seems like a refactoring rather than a behavioral fix).

https://github.com/llvm/llvm-project/pull/70829