[clang] [C++20][Modules] Add a test for field info assertion failure. (PR #155948)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 requested changes to this pull request.

I think we need to merge it. Why do you think we don't need to merge it?

https://github.com/llvm/llvm-project/pull/155948