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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> because we don't yet support non-zero initialization (as described in commit [5955a0f](https://github.com/llvm/llvm-project/commit/5955a0f9375a8c0b134eeb4a8de5155dcce7c94f))

I'm confused.  We support non-zero init, and there are tests for non-zero init in that commit. The commit message mentions dynamic initialization, but that's not non-zero; that's "requires code to run at program startup".

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