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

[Michael Park via cfe-commits]

mpark wrote:

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

oh goodness... I only just realized now that you meant that the decls should be merged. At the time I thought you were saying we should merge the PR 😂

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