[clang] [RFC][C++20][Modules] Fix crash when function and lambda inside loaded from different modules (PR #104512)

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

Quick update, all clang/cxx-library tests pass with the change but when I test it on my real project with modules I see crash or assert in [ASTDeclMerger::MergeDefinitionData](https://github.com/llvm/llvm-project/blob/main/clang/lib/Serialization/ASTReaderDecl.cpp#L2081) about faked lambda. I'm still creating small reproducer but it looks like it happens when lambda is inside class member function + some other conditions that I don't understand yet.

https://github.com/llvm/llvm-project/pull/104512