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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

We have some testing results in and it's causing a large-scale breakage. I **think** it's all coming from 2 root causes, but it's hard to know for sure because there are many thousands of broken targets.

The two problems I see are:
- invalid `no matching constructor for initialization of` errors for some particular type,
- a crash during template instantiations for some other particular type.

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?

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