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

[Harald van Dijk via cfe-commits]

https://github.com/hvdijk approved this pull request.

Looks good to me, thanks, aside from one typo in the release notes. Let me pre-emptively mark this as approved. Just to confirm, the warnings are also enabled by `-pedantic`? Is that worth mentioning in the release notes too? I'm fine if you want to leave that though.

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