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

[Kees Cook via cfe-commits]

kees wrote:

GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
Clang: https://github.com/llvm/llvm-project/issues/84565

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