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

[Nick Desaulniers via cfe-commits]

================
@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 %s -verify=c -fsyntax-only -fflex-array-extensions
+
+// The test checks that flexible array members do not emit warnings when
+// -fflex-array-extensions when used in a union or alone in a structure.
+
+struct already_hidden {
+	int a;
----------------
nickdesaulniers wrote:

```sh
$ cat clang/test/.clang-format
DisableFormat: true
```
:(

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