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

[Ilya Biryukov via cfe-commits]

ilya-biryukov 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.

Sadly, because this stopped further release testing, we don't know if it's more widespread. @alexfh will rerun the full testing with the fix and we'll report back on the progress again.

PS sorry for the delays, as Alex has mentioned there's been a lot of other background issues we were firefighting that slowed down testing for this particular issue.

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