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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

I do believe that the issue about ODR violation cannot be caused by this change itself. I suggest merging this PR as is and I'll take a look at the reproducer as soon as you create it. This PR fixes known issues with reproducers so I'm in doubt what makes this change so special that we cannot land it as normal PRs.

Said that I think we should invest into a tool to reduce issues in case of modules as we discussed. I also have a reproducer for C++ modules related issues that I cannot reduce for almost 2 weeks already. It slows us down a lot :(

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