[clang] [C++20] [Modules] [Serialization] Delay marking pending incomplete decl chains until the end of `finishPendingActions`. (PR #121245)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > +1. It is best to have more test case. Generally I reduce it by hand in this case.
> 
> Yeah, I did already spent quite a bit of effort trying to reduce the creduce result by hand but have not been successful so far 😕. I'll try again and report back. Would you still review the logic / whether the suggested solution seems sensible?

The solution looks neat to me. 

https://github.com/llvm/llvm-project/pull/121245