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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I have managed to get a small example, but it only crashes for me if I run it directly through our internal build system (not even if I use the exact same Clang binary).

There are a few things that might affect that:
- the names of the modules used internally are different; I have simplified them to save some typing,
- the flags we pass are different, but most of them should not matter (various macros only used internally, etc).

I suspect there are some hash tables somewhere and the order of something gets affected by those two factors above or something else and we end up loosing the repro.
I will need a little more time to produce the exact same command lines and module map inputs and reduce them further, but I wanted to share the actual code that produces the "inline function not defined" error; just in case folks would have ideas on what might go wrong in those examples before I get a full repro.

[friends.tgz](https://github.com/user-attachments/files/17869931/friends.tgz)



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