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

[Dmitry Polukhin via cfe-commits]

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@@ -0,0 +1,90 @@
+// RUN: rm -rf %t
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dmpolukhin wrote:

Please add a comment that the test results in `llvm_unreachable` in debug builds so debug build should be used to investigate problems with the test.

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