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

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

I don't have the necessary context to make sure the `SemaTemplateInstantiateDecl.cpp` is fully correct, but still wanted to share something that raised my eyebrows.

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