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

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> [Since the C standard requires the test case to be diagnosed](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548#c3)

I wonder what part of the spec discusses this?  Perhaps it's worth having the citation, just in case it helps revise the spec here one day.

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