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

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> Should I update this PR or create a new one?

Consider updating this one.  Worst case, if my fellow reviewers disagree with my suggestion, there's always `git reflog` for getting back to the initial version.

> GCC does not support this in either!

Do you have a bug on file with either us or them about this? Please cc me on those if so.

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