[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 could you please test the latest version on your sources?

It seems that I found generic enough solution for the problem when clang uses merged decl with no body for template instantiation. See changes in clang/lib/Sema/SemaTemplateInstantiateDecl.cpp.

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