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

[Michael Park via cfe-commits]

mpark wrote:

Hi @zixu-w, thanks for the ping. I can't take a look this week, but can on Monday. It would be great if I can look at a small repro though. I'll follow on the issue.

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