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

[Corentin Jabot via cfe-commits]

cor3ntin 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




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