[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:

@zixu-w @mpark given the original patch get backported to the release branch and the patch get reverted, I think we should revert the patch in the release branch too.

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