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

[John McCall via cfe-commits]

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@@ -7255,20 +7255,27 @@ void Sema::CheckCompletedCXXClass(Scope *S, CXXRecordDecl *Record) {
       CheckCompletedMemberFunction(MD);
   }
 
-  // ms_struct is a request to use the same ABI rules as MSVC.  Check
-  // whether this class uses any C++ features that are implemented
-  // completely differently in MSVC, and if so, emit a diagnostic.
-  // That diagnostic defaults to an error, but we allow projects to
-  // map it down to a warning (or ignore it).  It's a fairly common
-  // practice among users of the ms_struct pragma to mass-annotate
-  // headers, sweeping up a bunch of types that the project doesn't
-  // really rely on MSVC-compatible layout for.  We must therefore
-  // support "ms_struct except for C++ stuff" as a secondary ABI.
+  // {ms,gcc}_struct is a request to change ABI rules to either follow
+  // Microsoft or Itanium C++ ABI. However, even if these attributes are
+  // present, we do not layout classes following foreign ABI rules, but
+  // instead enter a special "compatibility mode", which only changes
+  // alignements of fundamental types and layout of bit fields.
----------------
rjmccall wrote:

```suggestion
  // alignments of fundamental types and layout of bit fields.
```

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