Revert "[C++20] [Modules] Don't import non-inline function bodies even if it...

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

This reverts commit 1ecbab56.

See the discussion in https://github.com/llvm/llvm-project/issues/86893.

The original commit receives too many complaints. Let's try to
workaround the issue to give better user experiences.