[clang] [C++20][Modules] Fix crash/compiler error due broken AST links (PR #123648)

[Haojian Wu via cfe-commits]

https://github.com/hokein commented:

> before this change RelatedDeclsMap was created only for canonical decl but in reality it has to be done for the definition of the function that does not always match the canonical decl.

I'm new to the module codebase, and it is not obvious to me. Could you elaborate on this? Perhaps you could use the provided test case to illustrate the distinction.


https://github.com/llvm/llvm-project/pull/123648