[clang] [C++20] [Modules] Fix may-be incorrect ADL for module local entities (PR #123931)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I'd like to land this given the scale of the patch and I want to fix it before 20.x and I may not be available in the next few days. 

https://github.com/llvm/llvm-project/pull/123931