[clang] [C++20] [Modules] Don't import function bodies from other module units even with optimizations (PR #71031)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I'd like to land this change later since the existing behavior violates our expectations for ABI.

https://github.com/llvm/llvm-project/pull/71031