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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Sorry, still trying to reproduce this and refraining from responding every day because I feel very close to getting a repro I can share. But that has been taking more time than I expected, please bear with me, I'm making progress.

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