[clang] [Clang][Sema]: Allow flexible arrays in unions and alone in structs (PR #84428)

[Kees Cook via cfe-commits]

kees wrote:

Ah, well, regardless, I think I found where the `StructuredList->setInitializedFieldInUnion` was actually missing, and then I could undo my zero-init-only and everything still appears fixed. Doing a full debug build test run now...

https://github.com/llvm/llvm-project/pull/84428