[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Bill Wendling via cfe-commits]

================
@@ -1155,15 +1159,14 @@ const FieldDecl *CodeGenFunction::FindCountedByField(const FieldDecl *FD) {
     return nullptr;
 
   auto GetNonAnonStructOrUnion = [](const RecordDecl *RD) {
-    while (RD && !RD->getDeclName())
-      if (const auto *R = dyn_cast<RecordDecl>(RD->getDeclContext()))
-        RD = R;
-      else
+    while (RD && RD->isAnonymousStructOrUnion()) {
+      const auto *R = dyn_cast<RecordDecl>(RD->getDeclContext());
+      if (!R)
----------------
bwendling wrote:

I think you could have:

```
int foo(void) {
  struct {
    int a, b;
  } x;

  ...
}
```

Though in the case of a struct with a flexible array member, probably not. Then again, you might be able to do this:

```
int foo(void) {
  struct {
    int count;
    int array[] __counted_by(count);
  } x = { 3 };
  ...
}
```

I just didn't want to make any assumptions. I can make this an assert if you'd like.

https://github.com/llvm/llvm-project/pull/76348