[clang] [clang][Sema] Add checks for validity of default ctor's class (PR #78898)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> I decided to not include tests, because our testing infrastructure is not ready to test that Clang doesn't crash without overspecifying the tests using `-verify` mode.

Could you elaborate a bit more on that? What is the exact problem with the testing infrastructure? Can we just add a separate test with the cases from the issues, perhaps without `-verify` at all?

https://github.com/llvm/llvm-project/pull/78898