[clang] Fix crash on defaulted comparison recovery in Sema and AST (PR #210668)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

This seems to largely be suppressing the symptoms - as far as I can tell this assertion is correct, and the bug is reaching this state.

I suspect something is going wrong earlier. My first guess is `DefineDefaultedComparison` failing to check for a complete class definition, but it may be earlier.

https://github.com/llvm/llvm-project/pull/210668