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

[Michael Park via cfe-commits]

mpark wrote:

> Did you try creduce/cvise (although using that with modules might be a bit challenging) It would be really nice to have a repro so that we have a test case

I did. It did help me reduce it significantly, but it still involves a few modules that are difficult to reduce. I still can make progress by running it on the modules themselves though. I agree it'd be really nice to have a test case.

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