[clang] Support [[guarded_by(mutex)]] attribute inside C struct (PR #94216)

[Pierre d'Herbemont via cfe-commits]

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@@ -29,6 +29,13 @@ struct LOCKABLE Mutex {};
 
 struct Foo {
   struct Mutex *mu_;
+  int  a_value GUARDED_BY(mu_);
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pdherbemont wrote:

done!

https://github.com/llvm/llvm-project/pull/94216