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

Recommit
https://github.com/llvm/llvm-project/commit/1ecbab56dcbb78268c8d19af34a50591f90b12a0

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

The new thing in this commit is to allow to import the function body
from instantiations if it is marked with always-inline. See the
discussion in https://github.com/llvm/llvm-project/issues/86893 for
details.