[clang] [Clang] Add detailed notes explaining why is_aggregate evaluates to false (PR #152488)

[Shamshura Egor via cfe-commits]

egorshamshura wrote:

@cor3ntin Hi!!
Can you help me, please? I've got a question about inherited constructors. Why ```Ctor->isInheritingConstructor()``` doesn't recognize inherited constructors? I think you can look on a test with S2 struct. (It should fail I guess) 

https://github.com/llvm/llvm-project/pull/152488