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

[Erich Keane via cfe-commits]

erichkeane wrote:

I'm pretty unmotivated here, I don't think it is worth allowing this all the time thanks to it not really being a conforming extension, and I don't think it is sufficiently worth creating a dialect over with the flag. 

>From what I can see, the GCC folks are equally as unmotivated, so unless that changes and they wish to do it, I don't think we'd want to do it here either.

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