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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

FYI, in our internal testing we also found a case when this PR fixes issue. Also we don't see any examples when it may break something so I think we should merge it ASAP because it fixes known issues and don't have any known regressions.

@alexfh what is the ETA for release testing?

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