[C++20] [Modules] [Windows] Collections for C++20 Modules for windows in clang

This is a meta issue collects the issues or missing features of C++20 modules for windows in clang.

Generally the number of such issues won't be a lot, thanks to the design of clang, many implementations of C++20 modules can be shared in different targets. So I guess it won't be too hard to fix these bugs and implement the missing part.

I don't mark this as "good first issue" as I feel "good first issue" is majorly for super trivial issues, as diagnostic improvements and so on. But I do think this is good opportunity for contributors if they are interested.

On the one hand, I don't have a windows machine. On the other hand, I am not interested in developing in windows. So I may not fix or implement this myself. But I will love to discuss implementation details if you have questions. And also, as said, the design of clang makes most of the implementation can be shared between different platforms. So this may be good entry point for contributors who are interested in modules.

Missing feature:

- [ ] https://github.com/llvm/llvm-project/issues/174064
- [ ] https://github.com/llvm/llvm-project/issues/174065

Ecosystem:

- [ ] [Clang-cl.exe support for C++ modules](https://discourse.llvm.org/t/clang-cl-exe-support-for-c-modules/72257)

Bugs:

- [ ] https://github.com/llvm/llvm-project/issues/173997
- [ ] https://github.com/llvm/llvm-project/issues/167638
- [ ] https://github.com/llvm/llvm-project/issues/160844
- [ ] https://github.com/llvm/llvm-project/issues/110751

The bug of list may not be completed, we can find it by: https://github.com/llvm/llvm-project/issues?q=state%3Aopen%20label%3A%22clang%3Amodules%22%20label%3Aplatform%3Awindows

Start by reviewing the linked missing-feature and bug issues, especially the clang-cl.exe modules discussion, then search the clang:modules and platform:windows labels for related work. This meta issue does not identify one implementation or test target; done means resolving the tracked Windows C++20 modules items and updating this checklist.

Written by the indexing model from the issue text.