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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> > I think the fix for this should be in DefineDefaultedComparison -- gating on the class definition being completed maybe?
> 
> Yes sir, I'm looking on this issue

"Sir" really isn't needed - I'm just an engineer :D

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