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

[Shamshura Egor via cfe-commits]

egorshamshura wrote:

> I am mostly afk, I will look at this over the weekend. maybe @AaronBallman can help

Ok, thanks!

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