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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

> Update: we've found a failure with `"function X has different bodies"` but my suspicion is that it's surfacing an existing issue rather than introducing a new one. I've managed to workaround it in our codebase by modularizing a bit more of the codebase.

Thank you for the update. I do think this issue is not directly related to this change and most probably just pre-existing issue detected. FYI, I recently disable ODR checks for header units like it was for named modules, see #111160 I don't know what happens for clang modules perhaps they also should be excluded from ODR check. In long run I would like to enable ODR check but with exclusion for common issue like set of visible overloaded functions visible that is hard to control and benign ODR violations.

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