[C++20] [Modules] Don't generate call to an imported module that dont init anything (#67638)

Close https://github.com/llvm/llvm-project/issues/56794

And see https://github.com/llvm/llvm-project/issues/67582 for a detailed
backgrond for the issue.

As required by the Itanium ABI, the module units have to generate the
initialization function. However, the importers are allowed to elide the
call to the initialization function if they are sure the initialization
function doesn't do anything.

This patch implemented this semantics.