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

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers commented:

Rather than have a `-f` flag to opt into this extension, I think instead you should just make it always available, then have tests that it can be used, but will trigger diagnostics under `-Wpedantic` since it's technically a language extension (IIUC).

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