[clang] [C++20][Modules] Fix crash when function and lambda inside loaded from different modules (PR #109167)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> > I've checked #111992 and it does fix the problem, so let's land it?
> 
> Yes, I would like to create a test case to don't regress this feature in future. I need to reduce libc++ `functional` header to something smaller.

Right, definitely +1 to adding a test case. I decided to hold off that comment until the PR moves away from the draft state, because I thought that's something that's on your mind anyway.

https://github.com/llvm/llvm-project/pull/109167