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

[Zixu Wang via cfe-commits]

zixu-w wrote:

Hi @mpark ! Unfortunately with this change we hit a crash in our bootstrap build with modules on macOS:
```
Assertion failed: (D && "missing definition for pattern of instantiated definition"), function hasAcceptableDefinition, file SemaType.cpp, line 9203.
```
Do you mind taking a look? Or temporarily revert this change.
(Compilation command and stack trace in Issue https://github.com/llvm/llvm-project/issues/126973)

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