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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

 With DeclContext visitor collectLambdas takes 0.03% or smaller, sometime I don't even see it in sampling profile.

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