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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

I'm sorry for the delay. Thanks for the ping.
I think this looks good.
Can you add tests for arrays? (arrays are aggregate https://eel.is/c++draft/dcl.init.aggr#1)

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