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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@dmpolukhin I am still confusing about the problem. I mean, why your previous patch will break the reproducer and why this patch can "fix" it? I feel the current patch is somewhat workaround. It's not your fault. The original codes are somewhat tricky already. But let's try to find methods to fix it more fundamentally.

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