[clang] [Clang] Fix missing diagnostic for non-standard layout type in `offsetof` (PR #65246)

[via cfe-commits]

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@@ -47,7 +47,11 @@ struct __libcpp_datasizeof {
   };
 #endif
 
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phyBrackets wrote:

Optional : Can you add a brief comment/note that why warnings are suppressed here? 
Mb something like : 

``` 
// Disable warnings related to the use of `offsetof` on non-standard layout types 
// TODO : Find a way to replace offsetof ?
```

https://github.com/llvm/llvm-project/pull/65246