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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@zixu-w and @ChuanqiXu9 Just want to clarify policies about changes reversion. As far as I understand it was one example in some private codebase with no reproducer publicly available. I can understand and completely agree if it breaks any existing test or llvm-build bot. But in this case I think it would be good to at least wait for the review. Such reversion without providing a reproducer does not allow to fix the issues or even verify that it is related to this PR. This PR fixes another crash with clear reproducers.

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