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

[Vlad Serebrennikov via cfe-commits]

Endilll 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.

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