[C++20] [Modules] Don't import non-inline function bodies even if it is marked as always_inline

Close https://github.com/llvm/llvm-project/issues/80949

Previously, I thought the always-inline function can be an exception to
enable optimizations as much as possible. However, it looks like it
breaks the ABI requirement we discussed later. So it looks better to not
import non-inline function bodies at all even if the function bodies are
marked as always_inline.

It doesn't produce regressions in some degree since the always_inline
still works in the same TU.