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

[Kees Cook via cfe-commits]

kees wrote:

For historical reference, the first version of this PR is visible here now: https://github.com/kees/llvm-project/commit/ce31f1d75f060b32e5dbc5756fe41cc8eaac83a6

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