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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I fully agree, both getting the reproducers and reducing them later is such a pain that we cannot be efficient without that tooling.
My team is going to start working on it soonish, we really feel the need. We'll make sure to loop you in to look for potential collaborations.

In the meantime, still making progress on this particular issues and still not done :(

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