[clang] [C++20][Modules] Load function body from the module that gives canonical decl (PR #111992)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> @ilya-biryukov I remember last time, the codes you shared to me actually based an open source project. I am wondering if you can open source some use for such projects (even for testing purpose). I think it will be pretty helpful to speedup the overall development process.

Yes, we should, the biggest problem right now is that our use of header modules is quite unique and is all internal.
There are technically features in Bazel that would allow to do something similar, but the BUILD files and the way to prepare it is too difficult for anyone to master. On top of that, we also have a ton of targeted opt-ins and opt-outs for specific libraries to avoid compiler bugs. Worse yet, even though the code I shared only had Absl bits, the actual usage patterns that cause the bugs are **always** in the internal code and it takes ages to distill it to those essential small examples.

That being said, we're going to spend significant effort next year to make sure this improves and we can get small reproducers fast and help to surface and fix any problems that show up. We're in a fragile state right now and want to get to more solid ground with our use of modules.

> @ilya-biryukov and @ChuanqiXu9 thank you for your help with the review, testing and reproducers. Very appreciated!

Thanks a lot for fixing this. We really appreciate that too. And I hope we can get to a point where we can give you the reproducers much faster some time next year.

https://github.com/llvm/llvm-project/pull/111992