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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

The latest version works like a charm for us, there aren't any bugs. I'll let @ChuanqiXu9 stamp the final approval for the code.
Let me know if you'll need more testing.

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