[C++20] [Modules] Don't generate strong function of a partition in importing modules

This solves the multiple defintiion issue
https://github.com/llvm/llvm-project/issues/54269.

I think the cause of the bug might be an oversight. We just forget to
edit this when implementing partitions. And it should be a good fix.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D121271