[clang] [clang] Stub out gcc_struct attribute (PR #71148)

[Aaron Ballman via cfe-commits]

================
@@ -3643,7 +3643,14 @@ def CFGuard : InheritableAttr, TargetSpecificAttr<TargetWindows> {
 def MSStruct : InheritableAttr {
   let Spellings = [GCC<"ms_struct">];
   let Subjects = SubjectList<[Record]>;
-  let Documentation = [Undocumented];
+  let Documentation = [MSStructDocs];
+  let SimpleHandler = 1;
+}
+
+def GCCStruct : InheritableAttr {
+  let Spellings = [GCC<"gcc_struct">];
+  let Subjects = SubjectList<[Record]>;
+  let Documentation = [GCCStructDocs];
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AaronBallman wrote:

```suggestion
  let Documentation = [MSStructDocs]; // Covers this attribute too.
```

https://github.com/llvm/llvm-project/pull/71148