[clang] [Clang] Explain why is_standard_layout evaluated to false (PR #143722)

[Samarth Narang via cfe-commits]

snarang181 wrote:

> Thanks a lot for working on that.
> 
> Can you add tests for all of these examples? https://eel.is/c++draft/class.prop#11
> 
> In particular
> 
> * If there are multiple direct base classes, a type is not a standard layout
> * If in the inheritance hierarchy, there are multiple classes with members, it is not _standard-layout_.
>   We have `CXXRecordDecl::hasDirectFields()` and `CXXRecordDecl::getStandardLayoutBaseWithFields` that should help

@cor3ntin, I added more inheritance related tests. Kindly review, thanks! 

https://github.com/llvm/llvm-project/pull/143722