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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@ilya-biryukov and @ChuanqiXu9 thank you for your help with the review, testing and reproducers. Very appreciated!

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