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

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> I tried my best to explain that it's not an issues of prettiness.

Well, ok, perhaps checking that there is actually no crash in clang can be tricky. But what I meant is, since there is no good way to check exit code, why can't we add `-verify` test? Yes, it will be checking errors that the patch didn't touch, but it is what mostly people do when adding clang tests and it will be +N test cases in a regular test base which not only ensure your change is correct, but the future ones too.

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