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

[Zixu Wang via cfe-commits]

zixu-w wrote:

Hi @dmpolukhin @ChuanqiXu9 @AaronBallman !

My apologies for the late reversion and not providing more context more promptly. To clarify, the issue does also reproduce in upstream and I've provided reproducing steps in https://github.com/llvm/llvm-project/issues/126973#issuecomment-2657949171. It was only caught in our downstream builds because we don't have the exact configuration to reproduce the problem in upstream builds and CIs.

And our downstream CI had been running into other issues in the past two weeks, which masked the problem and slowed us down in triaging. It also took me a while to understand the assertion and find which commit caused it.

Sorry again for the confusion. I can pick the reversion in the release branch. I'll help follow up with investigating and fixing #126973, and let me know if there's anything else I should do.

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