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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Some code got rearranged after our initial testing and we started seeing some new errors after this landed.
I have filed #122493 to track it, let's continue the discussion there.

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