[clang] [Clang] Implement diagnostics for why `std::is_standard_layout` is false (PR #144161)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

I'm happy with that (modulo nits) once @AaronBallman is happy 

https://github.com/llvm/llvm-project/pull/144161