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

[Dmitry Polukhin via cfe-commits]

https://github.com/dmpolukhin commented:

> The two problems I see are:
> 
> * invalid `no matching constructor for initialization of` errors for some particular type,

I think it is due to applying my changes too wide (i.e. to normal member functions and use first decl for them). I hope I fixed it in the latest version.

> * a crash during template instantiations for some other particular type.

In this case it is harder to make a reasonable guess but perhaps also the same root.

> And a few things that seem minor at first glance, but would also need some checking.
> 
> * some errors due to too long recursive template instantiations that we didn't see before (probably correct and coming from instantiating those bodies that were previously skipped?)
> * some compilation timeouts (probably just a flake or a slight increase that we should mitigate on our end).
> 
> And there may be some long tail that's not visible because these issues take all of my attention. I will try to get the reproducers for each of those. Or would it make sense to wait until you finish the review and do that for the new version?

Could you please try the latest version if it doesn't fix all issues, please create a reproducer to continue my investigation.

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