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

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

But then we (and GCC and MSVC) support the `[0]` extension syntax. So it's not like codegen is impossible.

https://godbolt.org/z/PGa9KWGxq

```c
union foo {
	int a;
	int b[]; // error: flexible array member 'b' in a union is not allowed
};
union bar {
	int a;
	int b[0]; // works just fine!
};
```

So to me, it seems like the standard can be amended to add the "or unions too" and already 3 implementations could generate valid code (GCC and clang would both need their semantic analyses relaxed here, but their codegen seems to work for the `[0]` extension already).

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