[clang] [API Notes] Attach API notes to forward declarations of tags (PR #149951)

[Gábor Horváth via cfe-commits]

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@@ -23,3 +23,8 @@ struct EscapableType { int value; };
 struct RefCountedTypeWithDefaultConvention {};
 inline void retain(RefCountedType *x) {}
 inline void release(RefCountedType *x) {}
+
+struct OpaqueRefCountedType;
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Xazax-hun wrote:

Could we add multiple forward declarations to make sure we do not duplicate the notes?

https://github.com/llvm/llvm-project/pull/149951